Add a flag to retain client developer messages (#38227)

## What changed

- Register `retain_client_developer_messages` as an under-development feature
  that is disabled by default.
- Expose the feature in the configuration schema.

GitOrigin-RevId: dc56c48fe79a9f288b7fe0a11d97af992d36797f

codex-rs/core/config.schema.json

@@ -696,6 +696,9 @@
             "responses_websockets_v2": {
               "type": "boolean"
             },
+            "retain_client_developer_messages": {
+              "type": "boolean"
+            },
             "rollout_budget": {
               "$ref": "#/definitions/FeatureToml_for_RolloutBudgetConfigToml"
             },
@@ -5335,6 +5338,9 @@
         "responses_websockets_v2": {
           "type": "boolean"
         },
+        "retain_client_developer_messages": {
+          "type": "boolean"
+        },
         "rollout_budget": {
           "$ref": "#/definitions/FeatureToml_for_RolloutBudgetConfigToml"
         },

codex-rs/features/src/lib.rs

@@ -279,6 +279,8 @@ pub enum Feature {
     PreventIdleSleep,
     /// Enable remote compaction v2 over the normal Responses API.
     RemoteCompactionV2,
+    /// Retain client-authored developer messages across compacted context windows.
+    RetainClientDeveloperMessages,
     /// Use Agent Identity for ChatGPT-authenticated sessions.
     UseAgentIdentity,
     /// Enable workspace dependency support.
@@ -1454,6 +1456,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::RetainClientDeveloperMessages,
+        key: "retain_client_developer_messages",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::UseAgentIdentity,
         key: "use_agent_identity",