Make the inline command hints navigable:
- Up/Down move a highlight through the matching commands (wraps around).
- Enter while a hint is open runs the highlighted command (routed through
  UserSubmitted as /<name>, so the app's _run_command handles it — not sent
  to the agent). Enter with hints closed still submits the raw text.
- Tab completes the input to the highlighted command's canonical name and
  stays in the field for typing args (was: always the first match).

CommandHints stays a non-focusable Static (renders the highlighted line with
reverse video); _PromptInput owns all key handling via a sibling reference.

Co-Authored-By: Claude <noreply@anthropic.com>

Show a non-interactive list of matching commands above the input box while
the user types a '/'-command (no whitespace yet). Tab completes the prefix to
the canonical name of the top match. Enter already routed '/...' through
_run_command, so submitting a typed command executes it instead of sending
it to the agent — unchanged.

- CommandRegistry.match(prefix): case-insensitive prefix match on name and
  aliases, exact match sorted first; empty prefix returns all.
- CommandHints(Static): purely visual, never takes focus, hidden by default.
- InputBox composes hints above the prompt and refreshes them on
  TextArea.Changed.

Co-Authored-By: Claude <noreply@anthropic.com>