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/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]