feat(tui): adding Shortcuts Overlay

[Ammaar-Alam]

(Continuing from the discussion in PR #2753 and issue #2730)

i think discoverability with a lot of the current shortcuts is lacking, thought something like this might be helpful.
didn't begin this from an issue beforehand, nor am i really sure if this overlay matches the style/tone that codex is going for, but i though smtg like this might be worth considering?

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(AI-written description bc am lazy 🦭)

• Rationale:
  • Reliability: Ctrl+/ and Ctrl+? vary across terminals; F1 is not present on many compact keyboards;
    Ctrl+\ sends SIGQUIT. Ctrl+O is consistent, unused elsewhere in the TUI, ergonomic, and works uniformly
    in the app.
  • Clarity: One obvious toggle for open/close, concise footer, and an overlay with clear sections
    reduces cognitive load.
• Tests & Snapshots:
  • Updated TUI snapshots to reflect the new footer and narrow‑width truncation. This includes
    chat composer and chatwidget snapshots (narrow widths now show truncated footer lines containing “⌃O
    shortcuts ⌃C” when space is constrained).
  • All tests pass locally after snapshot updates.
• Backwards Compatibility:
  • No configuration or API changes.
  • Ctrl+T transcript remains available; it’s just not advertised in the footer anymore (listed in the
    overlay instead).
  • Esc‑based backtrack behavior is unchanged and still appears contextually as “Esc edit prev”.

Summary

• Replaces unreliable shortcuts with a single, consistent toggle: Ctrl+O.
• Reorganizes overlay into intuitive groups and adds “Ctrl+V paste image”.
• Updates footer and overlay hints for clarity; removes confusing dynamic hints.
• Updates TUI snapshots to reflect the new footer and narrow‑width truncation; all tests pass.

Rationale

• Reliability: Ctrl+/ and Ctrl+? vary across terminals; F1 not available on many compact keyboards;
  Ctrl+\ sends SIGQUIT. Ctrl+O is consistent, unused elsewhere in the TUI, and ergonomic.
• Polish: One obvious key for open/close + clear sections avoids confusion.

User‑Facing Changes:

• Shortcuts Overlay: Ctrl+O opens/closes the shortcuts overlay anywhere; overlay can also be closed
  with Esc, q, or Ctrl+C.
• Overlay Groups:
  • Compose: Enter send; Shift+Enter/Ctrl+J newline; Ctrl+V paste image
  • Edit: Ctrl+U clear line; Ctrl+H delete left; Alt+Backspace delete word left; Ctrl+K clear to end
  • Navigate: Alt+←/→ word jump; ↑/↓ previous/next entry
  • Overlays: Ctrl+O shortcuts; Ctrl+T transcript
  • Backtrack: Esc Esc edit previous
• Footer: Shows “⏎ send”, newline hint (Shift+⏎ when enhanced keys supported, otherwise ⌃J), “⌃O
  shortcuts”, and “⌃C quit”. Transcript hint is removed from the footer to keep it focused; transcript
  remains available on Ctrl+T and listed in the overlay.
• Overlay Hints: Minimal hints (“↑/↓ scroll” and “Ctrl+O close” on the first line; “q quit” on the second)
  for the shortcuts overlay.
• Implementation Notes:
  • ChatComposer gains a use_shift_enter_hint boolean to preserve main’s “Shift+Enter vs Ctrl+J”
    newline hint selection.
  • Footer construction uses the shared key hint helpers for platform‑aware styling.
  • The shortcuts overlay content is created via Overlay::new_static_with_title_minimal_hints, with
    the grouped sections above.

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[Ammaar-Alam]

dare i mention 🫣 @edward-bayes @nornagon-openai @easong-openai
(pls review saur i can add a codex contribution to my resume for openai's swe internship before applying 😋 )