chore(auto-review) feature => stable (#19063)

## Summary
Turn on Auto Review

## Testing
- [x] Update unit tests

codex-rs/features/src/tests.rs

@@ -87,20 +87,11 @@ fn code_mode_only_requires_code_mode() {
 }
 
 #[test]
-fn guardian_approval_is_experimental_and_user_toggleable() {
+fn guardian_approval_is_stable_and_enabled_by_default() {
     let spec = Feature::GuardianApproval.info();
-    let stage = spec.stage;
 
-    assert!(matches!(stage, Stage::Experimental { .. }));
-    assert_eq!(stage.experimental_menu_name(), Some("Auto-review"));
-    assert_eq!(
-        stage.experimental_menu_description().map(str::to_owned),
-        Some(
-            "When Codex needs approval for higher-risk actions (e.g. sandbox escapes or blocked network access), route eligible approval requests to a carefully-prompted security reviewer subagent rather than blocking the agent on your input. This can consume significantly more tokens because it runs a subagent on every approval request.".to_string()
-        )
-    );
-    assert_eq!(stage.experimental_announcement(), None);
-    assert_eq!(Feature::GuardianApproval.default_enabled(), false);
+    assert_eq!(spec.stage, Stage::Stable);
+    assert_eq!(Feature::GuardianApproval.default_enabled(), true);
 }
 
 #[test]