Cleanup pass: --help, in-app help, post-rebuild message, unmodified-temp cleanup

Four post-Iteration-4 polish items surfaced by manual testing.

1. `--help` / `-h` CLI flag prints a usage banner (options +
   app-level commands + DSL grammar reference) and exits. Parse
   errors also print the banner to stderr.

2. `help` app-level command notes the same list of supported
   commands to the output panel -- a simple stand-in for the
   richer H3 help system, kept in sync with what's actually
   wired up.

3. The silent rebuild that runs when playground.db is missing
   now surfaces a system message in the output panel ("[ok]
   rebuild -- N tables, M rows reconstructed; ...") via a new
   initial_events plumbing. The user no longer wonders whether
   the .db was magically restored or whether anything happened
   on launch.

4. Unmodified empty temp projects (kind=Temp, project.yaml has
   tables: [] and relationships: []) are now auto-deleted when
   the user switches away (load / new / save as) or quits. This
   addresses the "launch app, load existing project, quit"
   pattern that was leaving an empty temp directory behind
   every time. Modified temps (with any user-created tables or
   relationships) are never auto-deleted; corrupted projects
   are also never auto-deleted (defensive default-to-false on
   yaml read/parse errors).

Tests: 338 passing (272 lib + 9 + 5 + 6 + 20 + 9 + 17),
0 failing, 0 skipped. Clippy clean.

src/app.rs

@@ -598,6 +598,10 @@ impl App {
         // 5 (export, import).
         match effective_input.as_str() {
             "quit" | "q" => return vec![Action::Quit],
+            "help" => {
+                self.note_help();
+                return Vec::new();
+            }
             "rebuild" => return vec![Action::PrepareRebuild],
             "save" => {
                 return self.handle_save_command(false);
@@ -1024,6 +1028,43 @@ impl App {
         }
     }
 
+    /// Note a flat list of currently-supported app-level
+    /// commands to the output panel.
+    ///
+    /// This is the simple Iteration-4 stand-in for a richer
+    /// help system (H3 in the requirements doc); it gives the
+    /// user a quick "what can I type?" reference that's
+    /// always accurate against the build they're running. As
+    /// new commands land, append them here.
+    fn note_help(&mut self) {
+        self.note_system("Supported commands:");
+        for line in [
+            "  quit / q              — exit",
+            "  help                  — this list",
+            "  mode simple|advanced  — switch input mode",
+            "  rebuild               — rebuild .db from project.yaml + data/ (with confirmation)",
+            "  save                  — save current temp project under a name",
+            "  save as               — copy current project to a new name/path",
+            "  new                   — close current, start a fresh temp project",
+            "  load                  — open the project picker",
+            "DSL data commands (in simple mode):",
+            "  create table <T> with pk [<col>:<type>...]",
+            "  drop table <T>",
+            "  add column [to table] <T>: <col> (<type>)",
+            "  add 1:n relationship [as <name>] from <P>.<col> to <C>.<col>",
+            "    [on delete <action>] [on update <action>] [--create-fk]",
+            "  drop relationship <name>",
+            "  insert into <T> [(cols)] [values] (vals)",
+            "  update <T> set <c>=<v>... where <c>=<v> | --all-rows",
+            "  delete from <T> where <c>=<v> | --all-rows",
+            "  show table <T>",
+            "  show data <T>",
+            "Types: text, int, real, decimal, bool, date, datetime, blob, serial, shortid",
+        ] {
+            self.note_system(line);
+        }
+    }
+
     fn handle_mode_command(&mut self, raw: &str) {
         let arg = raw.strip_prefix("mode").unwrap_or(raw).trim();
         match arg {

src/cli.rs

@@ -20,8 +20,45 @@ pub struct Args {
     /// this path (L1, ADR-0015 §1). Mutually exclusive with
     /// `--resume` once that lands.
     pub project_path: Option<PathBuf>,
+    /// `--help` / `-h`: print usage to stdout and exit. The
+    /// runtime checks this flag before doing any other work.
+    pub help: bool,
 }
 
+/// Usage banner printed by `--help`. Kept as one block so the
+/// formatting is reviewable on its own.
+pub const HELP_TEXT: &str = "\
+rdbms-playground — a TUI playground for relational database concepts
+
+Usage:
+    rdbms-playground [options] [<project-path>]
+
+Arguments:
+    <project-path>           Path to an existing project directory.
+                             Without this, a fresh auto-named temp
+                             project is created in the data dir.
+
+Options:
+    -h, --help               Print this help and exit.
+        --theme <light|dark> Override theme (default: auto-detect).
+        --data-dir <PATH>    Use PATH as the data root instead of
+                             the OS-standard location for this run.
+        --log-file <PATH>    Write tracing output to PATH.
+
+App-level commands (typed inside the app, available in both modes):
+    quit / q                 Exit cleanly.
+    mode simple|advanced     Switch input mode.
+    help                     Show this list of commands in-app.
+    save                     Save the current temp project under a
+                             chosen name (or `save as` to copy a
+                             named project to a new location).
+    save as                  Always prompt for a target name/path.
+    new                      Close current, create a fresh temp.
+    load                     Open the project picker.
+    rebuild                  Rebuild playground.db from project.yaml
+                             + data/, with confirmation.
+";
+
 #[derive(Debug, thiserror::Error)]
 pub enum ArgsError {
     #[error("missing value for --{0}")]
@@ -54,9 +91,13 @@ impl Args {
         let mut log_path = env::var_os("RDBMS_PLAYGROUND_LOG_FILE").map(PathBuf::from);
         let mut data_dir: Option<PathBuf> = None;
         let mut project_path: Option<PathBuf> = None;
+        let mut help = false;
         let mut iter = iter.into_iter().map(Into::into);
         while let Some(arg) = iter.next() {
             match arg.as_str() {
+                "--help" | "-h" => {
+                    help = true;
+                }
                 "--theme" => {
                     let value = iter.next().ok_or(ArgsError::MissingValue("theme"))?;
                     theme = match value.as_str() {
@@ -98,6 +139,7 @@ impl Args {
             log_path,
             data_dir,
             project_path,
+            help,
         })
     }
 }
@@ -203,6 +245,18 @@ mod tests {
         assert!(matches!(err, ArgsError::MultiplePaths { .. }), "got: {err:?}");
     }
 
+    #[test]
+    fn help_flag_long_form_sets_help() {
+        let args = Args::parse(["--help"]).unwrap();
+        assert!(args.help);
+    }
+
+    #[test]
+    fn help_flag_short_form_sets_help() {
+        let args = Args::parse(["-h"]).unwrap();
+        assert!(args.help);
+    }
+
     #[test]
     fn unknown_double_dash_flag_errors_even_with_positional() {
         // Make sure the path-vs-flag distinction is robust:

src/main.rs

@@ -1,6 +1,6 @@
 use std::process::ExitCode;
 
-use rdbms_playground::cli::Args;
+use rdbms_playground::cli::{Args, HELP_TEXT};
 use rdbms_playground::{logging, runtime};
 
 fn main() -> ExitCode {
@@ -8,10 +8,16 @@ fn main() -> ExitCode {
         Ok(args) => args,
         Err(e) => {
             eprintln!("rdbms-playground: {e}");
+            eprintln!("\n{HELP_TEXT}");
             return ExitCode::from(2);
         }
     };
 
+    if args.help {
+        print!("{HELP_TEXT}");
+        return ExitCode::SUCCESS;
+    }
+
     if let Err(e) = logging::init(args.log_path.as_deref()) {
         eprintln!("rdbms-playground: failed to initialise logging: {e:#}");
         return ExitCode::FAILURE;

src/project/mod.rs

@@ -346,6 +346,37 @@ impl Project {
         self.kind
     }
 
+    /// Is this an auto-named temp project that the user has
+    /// not modified?
+    ///
+    /// Used to clean up the inevitable accumulation of
+    /// auto-named temp directories left behind when the user
+    /// launches the app, immediately loads another project
+    /// (or quits without doing anything), and never returns
+    /// to the temp.
+    ///
+    /// "Unmodified" is defined as: kind is Temp AND
+    /// `project.yaml` lists no tables and no relationships.
+    /// The user-visible schema is what counts — show queries
+    /// only append to history.log and don't trip this check.
+    /// Errors reading or parsing the YAML default to "not
+    /// unmodified" (false), so a corrupted project is never
+    /// auto-deleted.
+    #[must_use]
+    pub fn is_unmodified_temp(&self) -> bool {
+        if !matches!(self.kind, ProjectKind::Temp) {
+            return false;
+        }
+        let yaml_path = self.path.join(PROJECT_YAML);
+        let Ok(body) = fs::read_to_string(&yaml_path) else {
+            return false;
+        };
+        let Ok(snapshot) = crate::persistence::parse_schema(&body) else {
+            return false;
+        };
+        snapshot.tables.is_empty() && snapshot.relationships.is_empty()
+    }
+
     /// Path to the SQLite database for this project. Always
     /// `<project>/playground.db`.
     #[must_use]

src/runtime.rs

@@ -64,24 +64,36 @@ pub async fn run(args: Args) -> Result<()> {
     let db_existed = db_path.exists();
     let database = Database::open_with_persistence(db_path.as_path(), persistence)
         .context("open database")?;
-    if !db_existed
-        && let Err(e) = database
-            .rebuild_from_text(project_path.clone(), None)
-            .await
-    {
-        // The terminal is still in cooked mode here (we haven't
-        // entered the alternate screen yet), so writing to
-        // stderr lands directly in the user's shell. Drop the
-        // project to release the lock first.
-        drop(project);
-        if matches!(
-            e,
-            DbError::PersistenceFatal { .. } | DbError::RebuildRowFailed { .. }
-        ) {
-            eprintln!("rdbms-playground: {}", e.friendly_message());
-            return Ok(());
+    let mut initial_events: Vec<AppEvent> = Vec::new();
+    if !db_existed {
+        match database.rebuild_from_text(project_path.clone(), None).await {
+            Ok(()) => {
+                // Surface the silent rebuild as a system note
+                // so the user sees that the .db was
+                // reconstructed rather than wondering whether
+                // anything happened.
+                let summary = summarize_project(&project_path).unwrap_or_else(|_| {
+                    "rebuilt playground.db from project.yaml + data/".to_string()
+                });
+                initial_events.push(AppEvent::RebuildSucceeded { summary });
+            }
+            Err(e) => {
+                // The terminal is still in cooked mode here
+                // (we haven't entered the alternate screen
+                // yet), so writing to stderr lands directly
+                // in the user's shell. Drop the project to
+                // release the lock first.
+                drop(project);
+                if matches!(
+                    e,
+                    DbError::PersistenceFatal { .. } | DbError::RebuildRowFailed { .. }
+                ) {
+                    eprintln!("rdbms-playground: {}", e.friendly_message());
+                    return Ok(());
+                }
+                return Err(anyhow::anyhow!(e.friendly_message())).context("rebuild from text");
+            }
         }
-        return Err(anyhow::anyhow!(e.friendly_message())).context("rebuild from text");
     }
 
     let mut terminal = setup_terminal().context("setup terminal")?;
@@ -95,6 +107,7 @@ pub async fn run(args: Args) -> Result<()> {
         },
         display_name,
         project_is_temp,
+        initial_events,
     )
     .await;
     if let Err(e) = teardown_terminal(&mut terminal) {
@@ -149,6 +162,7 @@ async fn run_loop(
     mut session: Session,
     project_display_name: String,
     project_is_temp: bool,
+    initial_events: Vec<AppEvent>,
 ) -> Result<Option<String>> {
     let (event_tx, mut event_rx) = mpsc::channel::<AppEvent>(EVENT_CHANNEL_CAPACITY);
     let reader_handle = spawn_event_reader(event_tx.clone());
@@ -157,6 +171,14 @@ async fn run_loop(
     app.project_name = Some(project_display_name);
     app.project_is_temp = project_is_temp;
 
+    // Send any startup events (e.g., the system-message form
+    // of "rebuilt from text on missing .db") so they're
+    // dispatched through the normal event path and end up in
+    // the output panel before the user types anything.
+    for event in initial_events {
+        let _ = event_tx.send(event).await;
+    }
+
     // Seed the table list with whatever the database currently
     // shows. For a fresh in-memory DB this is empty, but doing
     // it explicitly means file-backed databases (track 2) will
@@ -251,6 +273,31 @@ async fn run_loop(
 
     let _ = tokio::time::timeout(SHUTDOWN_GRACE, reader_handle).await;
 
+    // Auto-delete the active project on quit if it's an
+    // unmodified temp — same rule as on project switch (see
+    // perform_switch). Captures the path first, drops the
+    // project (releasing the lock), then removes the dir.
+    let cleanup_on_quit: Option<std::path::PathBuf> = session
+        .project
+        .as_ref()
+        .and_then(|p| p.is_unmodified_temp().then(|| p.path().to_path_buf()));
+    let _ = session.database.take();
+    let _ = session.project.take();
+    if let Some(stale) = cleanup_on_quit {
+        if let Err(e) = std::fs::remove_dir_all(&stale) {
+            tracing::warn!(
+                path = %stale.display(),
+                error = %e,
+                "could not clean up unmodified temp project on quit",
+            );
+        } else {
+            tracing::info!(
+                path = %stale.display(),
+                "cleaned up unmodified temp project on quit",
+            );
+        }
+    }
+
     info!("event loop exited");
     Ok(app.fatal_message.clone())
 }
@@ -343,6 +390,15 @@ async fn perform_switch(
         copy_project(&src, dst).map_err(|e| e.to_string())?;
     }
 
+    // Capture cleanup info from the OUTGOING project before
+    // we drop it: if it was an unmodified empty temp, we
+    // delete its directory after the switch so the data dir
+    // doesn't accumulate empty scratch projects.
+    let outgoing_cleanup_path: Option<std::path::PathBuf> =
+        session.project.as_ref().and_then(|p| {
+            p.is_unmodified_temp().then(|| p.path().to_path_buf())
+        });
+
     // Drop current project + database BEFORE opening the new
     // ones, releasing the old lock and stopping the old
     // worker. Required for the "load my own current project"
@@ -351,6 +407,23 @@ async fn perform_switch(
     let _ = session.database.take();
     let _ = session.project.take();
 
+    // The outgoing project's lock is now released; it's
+    // safe to remove its directory if it was unmodified.
+    if let Some(stale) = outgoing_cleanup_path {
+        if let Err(e) = std::fs::remove_dir_all(&stale) {
+            tracing::warn!(
+                path = %stale.display(),
+                error = %e,
+                "could not clean up unmodified temp project",
+            );
+        } else {
+            tracing::info!(
+                path = %stale.display(),
+                "cleaned up unmodified temp project on switch",
+            );
+        }
+    }
+
     // Open the destination project.
     let new_project = match &req {
         SwitchRequest::Load { .. } | SwitchRequest::SaveAs { .. } => {