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.

docs/tui-chat-composer.md

@@ -49,6 +49,31 @@ The solution is to detect paste-like _bursts_ and buffer them into a single expl
 - After handling the key, `sync_popups()` runs so popup visibility/filters stay consistent with the
   latest text + cursor.
 
+## Config gating for reuse
+
+`ChatComposer` now supports feature gating via `ChatComposerConfig`
+(`codex-rs/tui/src/bottom_pane/chat_composer.rs`). The default config preserves current chat
+behavior.
+
+Flags:
+
+- `popups_enabled`
+- `slash_commands_enabled`
+- `image_paste_enabled`
+
+Key effects when disabled:
+
+- When `popups_enabled` is `false`, `sync_popups()` forces `ActivePopup::None`.
+- When `slash_commands_enabled` is `false`, the composer does not treat `/...` input as commands.
+- When `slash_commands_enabled` is `false`, the composer does not expand custom prompts in
+  `prepare_submission_text`.
+- When `slash_commands_enabled` is `false`, slash-context paste-burst exceptions are disabled.
+- When `image_paste_enabled` is `false`, file-path paste image attachment is skipped.
+
+Built-in slash command availability is centralized in
+`codex-rs/tui/src/bottom_pane/slash_commands.rs` and reused by both the composer and the command
+popup so gating stays in sync.
+
 ## Submission flow (Enter/Tab)
 
 There are multiple submission paths, but they share the same core rules: