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/friendly/strings/en-US.yaml

@@ -204,6 +204,9 @@ help:
                                  project's stored mode. Without it, the
                                  project's last-used mode is restored
                                  (default: simple).
+            --demo               Demonstration mode: show on-screen badges
+                                 for otherwise-invisible keys (Tab, Enter,
+                                 ...) — for screencasts and live teaching.
 
     App-level commands (typed inside the app, available in both modes):
         quit                     Exit cleanly.