Navi Code TUI: inline slash-command hints + Tab completion ...

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>