Register the Guardian thread context feature flag (#42529)

## What changed

- Add the disabled, under-development `guardian_thread_context` feature flag for selecting thread-owned context for Guardian reviewers.
- Expose the boolean setting in the generated configuration schema.
- Verify default, base configuration, and profile override resolution.

GitOrigin-RevId: c4878be96d12f4ccd84356df15d3e55f13ac141f

codex-rs/core/config.schema.json

@@ -774,6 +774,9 @@
             "guardian_reuse_parent_compaction": {
               "type": "boolean"
             },
+            "guardian_thread_context": {
+              "type": "boolean"
+            },
             "guardianv2": {
               "$ref": "#/definitions/FeatureToml_for_GuardianV2ConfigToml"
             },
@@ -6122,6 +6125,9 @@
         "guardian_reuse_parent_compaction": {
           "type": "boolean"
         },
+        "guardian_thread_context": {
+          "type": "boolean"
+        },
         "guardianv2": {
           "$ref": "#/definitions/FeatureToml_for_GuardianV2ConfigToml"
         },

codex-rs/features/src/lib.rs

@@ -299,6 +299,9 @@ pub enum Feature {
     SendAsyncMessage,
     /// Enable automatic review for approval prompts.
     GuardianApproval,
+    /// Select thread-owned context for both Guardian reviewers.
+    /// Config-only until reviewer wiring lands; read from the thread's fixed feature set.
+    GuardianThreadContext,
     /// Reuse encrypted parent compaction when restarting Guardian review sessions.
     GuardianReuseParentCompaction,
     /// Include completed node_repl or cua_repl Code Mode responses in Guardian reviews.
@@ -1519,6 +1522,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::GuardianThreadContext,
+        key: "guardian_thread_context",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::GuardianReuseParentCompaction,
         key: "guardian_reuse_parent_compaction",

codex-rs/features/src/tests.rs

@@ -162,6 +162,44 @@ fn guardian_v2_feature_config_preserves_boolean_toggle() {
     );
 }
 
+#[test]
+fn guardian_thread_context_resolves_boolean_config_and_profile_overrides() {
+    for (base, profile, enabled) in [
+        ("", "", false),
+        ("guardian_thread_context = false", "", false),
+        ("guardian_thread_context = true", "", true),
+        (
+            "guardian_thread_context = true",
+            "guardian_thread_context = false",
+            false,
+        ),
+        (
+            "guardian_thread_context = false",
+            "guardian_thread_context = true",
+            true,
+        ),
+    ] {
+        let base = toml::from_str::<FeaturesToml>(base).expect("base features");
+        let profile = toml::from_str::<FeaturesToml>(profile).expect("profile features");
+        let features = Features::from_sources(
+            FeatureConfigSource {
+                features: Some(&base),
+                ..Default::default()
+            },
+            FeatureConfigSource {
+                features: Some(&profile),
+                ..Default::default()
+            },
+            FeatureOverrides::default(),
+        );
+        let mut expected = Features::with_defaults();
+        if enabled {
+            expected.enable(Feature::GuardianThreadContext);
+        }
+        assert_eq!(features.enabled_features(), expected.enabled_features());
+    }
+}
+
 #[test]
 fn guardian_v2_feature_config_deserializes_classifier_and_transcript_settings() {
     let features: FeaturesToml = toml::from_str(