feat(cli): --demo demonstration mode flag + app plumbing (#22, ADR-0047 D1)

Add `--demo` (and the RDBMS_PLAYGROUND_DEMO env fallback) to enter
demonstration mode, threaded onto App.demo_mode through run_loop —
mirrors the --no-undo plumbing. Off by default, zero footprint when
off. The --help line advertises only the visible keystroke badges;
the Ctrl+] caption trigger is kept low-profile (ADR-0047 D6 updated).

Phase A of ADR-0047; behaviour (badges/captions) lands in B and C.

src/runtime.rs

@@ -216,6 +216,9 @@ pub async fn run(args: Args) -> Result<()> {
     let db_existed = db_path.exists();
     // Undo is on unless `--no-undo` (ADR-0006 Amendment 1).
     let undo_enabled = !args.no_undo;
+    // Demonstration mode under `--demo` / `RDBMS_PLAYGROUND_DEMO`
+    // (ADR-0047). Off by default; threaded onto the `App` in run_loop.
+    let demo_mode = args.demo;
     let database =
         Database::open_with_persistence_and_undo(db_path.as_path(), persistence, undo_enabled)
             .context("open database")?;
@@ -273,6 +276,7 @@ pub async fn run(args: Args) -> Result<()> {
         initial_events,
         undo_enabled,
         resolved_mode,
+        demo_mode,
     )
     .await;
     if let Err(e) = teardown_terminal(&mut terminal) {
@@ -331,6 +335,7 @@ async fn run_loop(
     initial_events: Vec<AppEvent>,
     undo_enabled: bool,
     initial_mode: crate::mode::Mode,
+    demo_mode: bool,
 ) -> Result<Option<String>> {
     let (event_tx, mut event_rx) = mpsc::channel::<AppEvent>(EVENT_CHANNEL_CAPACITY);
     let reader_handle = spawn_event_reader(event_tx.clone());
@@ -339,6 +344,8 @@ async fn run_loop(
     app.project_name = Some(project_display_name);
     app.project_is_temp = project_is_temp;
     app.undo_enabled = undo_enabled;
+    // ADR-0047: enable the demo overlays for this session under `--demo`.
+    app.demo_mode = demo_mode;
     // Start in the resolved input mode (ADR-0015 mode-restore
     // amendment, issue #14): `--mode` > stored project mode >
     // default. `Persistence` already carries the same value, so the