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/core/src/mcp/mod.rs

@@ -97,6 +97,7 @@ fn codex_apps_mcp_server_config(config: &Config, auth: Option<&CodexAuth>) -> Mc
         tool_timeout_sec: None,
         enabled_tools: None,
         disabled_tools: None,
+        scopes: None,
     }
 }