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/app.rs

@@ -368,6 +368,14 @@ pub struct App {
     /// flag; the `undo` / `redo` commands then report undo is off
     /// rather than emitting a prepare action.
     pub undo_enabled: bool,
+    /// Whether **demonstration mode** is active this session (ADR-0047,
+    /// issue #22). `true` under `--demo` / `RDBMS_PLAYGROUND_DEMO`. When
+    /// off (the default) none of the demo key handling or overlay
+    /// rendering runs — zero footprint. When on, otherwise-invisible
+    /// keys raise a transient badge (set by the runtime, see
+    /// `demo_badge`) and `Ctrl+]` drives the stealth step-caption
+    /// buffer (`demo_caption` / `demo_capturing`).
+    pub demo_mode: bool,
     /// The DSL → SQL teaching echo (ADR-0038) for the command currently
     /// being rendered: set from the success event just before its handler
     /// runs, consumed by `note_ok_summary` (which pushes it beneath
@@ -512,6 +520,9 @@ impl App {
             // Undo is on by default; the runtime flips this off for
             // a `--no-undo` session (ADR-0006 Amendment 1).
             undo_enabled: true,
+            // Demo mode is off by default; the runtime flips it on for
+            // a `--demo` session (ADR-0047).
+            demo_mode: false,
             pending_echo: None,
         }
     }