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"
         },