Promote TUI unified mentions in composer to default mentions feature (#27499)

## Summary

This PR promotes Mentions 2.0 (unified TUI mention popup) to stable and
enables it by default.

- Keep `mentions_v2` as a temporary rollback path to the legacy split
popups (`--disable mentions_v2`).
- Add feature-default and snapshot coverage for the default experience.

## Prior work

- [#19068 — Unified mentions in
TUI](https://github.com/openai/codex/pull/19068)
- [#22375 — Use plugin/list to get plugins for
mentions](https://github.com/openai/codex/pull/22375)
- [#23363 — Unified mentions tweaks and rendering
polish](https://github.com/openai/codex/pull/23363)

## Test plan

- Launch Codex without any feature overrides.
- Type `@` in the TUI composer.
- Confirm the unified mentions menu opens and displays filesystem,
plugin, and skill results.

codex-rs/features/src/tests.rs

@@ -310,9 +310,9 @@ fn auth_elicitation_is_under_development() {
 }
 
 #[test]
-fn mentions_v2_is_under_development_and_disabled_by_default() {
-    assert_eq!(Feature::MentionsV2.stage(), Stage::UnderDevelopment);
-    assert_eq!(Feature::MentionsV2.default_enabled(), false);
+fn mentions_v2_is_stable_and_enabled_by_default() {
+    assert_eq!(Feature::MentionsV2.stage(), Stage::Stable);
+    assert_eq!(Feature::MentionsV2.default_enabled(), true);
     assert_eq!(feature_for_key("mentions_v2"), Some(Feature::MentionsV2));
 }