Reuse parent compactions in Guardian review sessions (#37513)

## What changed

- Add the under-development `guardian_reuse_parent_compaction` feature.
- Restart Guardian review sessions after parent history rewrites and seed them with the latest encrypted compaction that has a response item ID.
- Keep the existing reviewer when a rewritten history has no reusable compaction, preserving authorization and restriction context held by that session.

## Testing

- Cover reuse-key invalidation and compaction eligibility.
- Verify review-session behavior across parent compaction and a subsequent summary-free history reset.

GitOrigin-RevId: 891805d3c3dca34ddda6e3bfc5097be4ff164267

codex-rs/features/src/lib.rs

@@ -247,6 +247,8 @@ pub enum Feature {
     DefaultModeRequestUserInput,
     /// Enable automatic review for approval prompts.
     GuardianApproval,
+    /// Reuse encrypted parent compaction when restarting Guardian review sessions.
+    GuardianReuseParentCompaction,
     /// Enable Guardian V2 automatic approval reviews.
     GuardianV2,
     /// Enable persisted thread goals and automatic goal continuation.
@@ -1365,6 +1367,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::GuardianReuseParentCompaction,
+        key: "guardian_reuse_parent_compaction",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::GuardianV2,
         key: "guardianv2",