Register the Guardian V2 feature flag (#35049)

## What changed

- Add `GuardianV2` to the feature registry for automatic approval reviews.
- Expose it as `features.guardianv2` in the configuration schema.
- Keep the under-development feature disabled by default.

GitOrigin-RevId: 92fa107e3ff1396a75a28a071353743a7bb48c43

codex-rs/features/src/lib.rs

@@ -225,6 +225,8 @@ pub enum Feature {
     DefaultModeRequestUserInput,
     /// Enable automatic review for approval prompts.
     GuardianApproval,
+    /// Enable Guardian V2 automatic approval reviews.
+    GuardianV2,
     /// Enable persisted thread goals and automatic goal continuation.
     Goals,
     /// Add current context-window metadata to model-visible context.
@@ -1276,6 +1278,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::GuardianV2,
+        key: "guardianv2",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::Goals,
         key: "goals",