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.

docs/adr/0047-demonstration-overlay-layer.md

@@ -276,10 +276,15 @@ from `IndicatorDebounce`) pair — not an inconsistency.
 The CLI banner (`help.cli_banner` in `en-US.yaml`) gains a `--demo`
 line. User-facing wording obeys the house rules (no engine name, no
 "DSL"): *"Demonstration mode — show on-screen badges for otherwise-
-invisible keys (Tab, Enter, …) and enable scripted step captions, for
-screencasts and live teaching."* Badge labels and the `[…]` chrome are
-fixed ASCII, not localised; caption content is author-supplied free
-text and likewise not a catalog string.
+invisible keys (Tab, Enter, …), for screencasts and live teaching."*
+
+The help text **deliberately mentions only the visible badges, not the
+`Ctrl+]` step-caption mechanism** (user decision): the caption trigger
+stays low-profile, true to #22's "secret trigger" framing — a cast
+author or lesson script knows it; a casual `--help` reader is not
+pointed at it. Badge labels and the `[…]` chrome are fixed ASCII, not
+localised; caption content is author-supplied free text and likewise
+not a catalog string.
 
 ## Alternatives considered