Add composer config and shared menu surface helpers (#9891)

Centralize built-in slash-command gating and extract shared menu-surface
helpers.

- Add bottom_pane::slash_commands and reuse it from composer + command
popup.
- Introduce ChatComposerConfig + shared menu surface rendering without
changing default behavior.

codex-rs/tui/src/bottom_pane/mod.rs

@@ -59,6 +59,7 @@ mod list_selection_view;
 mod prompt_args;
 mod skill_popup;
 mod skills_toggle_view;
+mod slash_commands;
 pub(crate) use footer::CollaborationModeIndicator;
 pub(crate) use list_selection_view::SelectionViewParams;
 mod feedback_view;