Add stage field for experimental flags. (#10793)

- [x] Add stage field for experimental flags.

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -1055,18 +1055,39 @@ pub struct ExperimentalFeatureListParams {
     pub limit: Option<u32>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub enum ExperimentalFeatureStage {
+    /// Feature is available for user testing and feedback.
+    Beta,
+    /// Feature is still being built and not ready for broad use.
+    UnderDevelopment,
+    /// Feature is production-ready.
+    Stable,
+    /// Feature is deprecated and should be avoided.
+    Deprecated,
+    /// Feature flag is retained only for backwards compatibility.
+    Removed,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct ExperimentalFeature {
     /// Stable key used in config.toml and CLI flag toggles.
-    pub flag_name: String,
+    pub name: String,
+    /// Lifecycle stage of this feature flag.
+    pub stage: ExperimentalFeatureStage,
     /// User-facing display name shown in the experimental features UI.
-    pub display_name: String,
+    /// Null when this feature is not in beta.
+    pub display_name: Option<String>,
     /// Short summary describing what the feature does.
-    pub description: String,
+    /// Null when this feature is not in beta.
+    pub description: Option<String>,
     /// Announcement copy shown to users when the feature is introduced.
-    pub announcement: String,
+    /// Null when this feature is not in beta.
+    pub announcement: Option<String>,
     /// Whether this feature is currently enabled in the loaded config.
     pub enabled: bool,
     /// Whether this feature is enabled by default.