feat(demo): Ctrl-G as a demo-mode F1 alias for casts (ADR-0047 Amendment 1)

The contextual hint overlay (ADR-0053) opens on F1, but F1 is an escape
sequence the autocast recorder can't emit — so casts (and presenter /
teacher sessions) couldn't trigger the most teaching-relevant overlay.

In demo mode only, Ctrl-G now aliases F1: it runs the same hint logic and
badges AS [F1], so a recording is visually identical to a real F1 press.
Ctrl-G is the only fit — Ctrl+digit (e.g. Ctrl-1) isn't encodable in a
legacy terminal (arrives as a bare `1`), and the kitty protocol that would
encode it needs escape sequences autocast can't send (and the app doesn't
enable keyboard-enhancement flags). Demo-gated, so the shipped keymap
stays F1-only; outside demo mode Ctrl-G is inert.

- app.rs: hint_key guard gains the demo-gated Ctrl-G disjunct;
  demo_badge_label maps Ctrl-G -> [F1]; 3 Tier-1 tests + badge assertion.
- ADR-0047 Amendment 1 + README index; also removed two stray
  </content> / </invoke> lines accidentally committed in the ADR file.

docs: drop three more stale "deferred" entries from CLAUDE.md — readline
shortcuts (I1b, ADR-0049), tab completion (I3), and syntax highlighting
(I4) are all implemented; only multi-line input (I1) remains open.

CLAUDE.md

@@ -347,11 +347,8 @@ not yet implemented:
 - **Session log + Markdown export** (V4): the bigger UX
   project — scrollable session journal, smart structure
   rendering, save-as-markdown.
-- **Readline shortcuts** (I1b): Ctrl-A/Ctrl-E, Ctrl-W/Ctrl-K/
-  Ctrl-U.
 - **Multi-line input** (I1): Enter inserts newline,
   Ctrl-Enter submits.
-- **Tab completion** (I3), **syntax highlighting** (I4).
 - **ER diagram export** (V3).
 - **Full TT5** (CI): the pipeline is live (see the CI decision
   above / `docs/ci/adr/`), but "all tiers on all OSes" isn't