chore(config) personality as a feature (#10116)

## Summary
Sets up an explicit Feature flag for `/personality`, so users can now
opt in to it via `/experimental`. #10114 also updates the config

## Testing
- [x] Tested locally

codex-rs/core/src/features.rs

@@ -119,6 +119,8 @@ pub enum Feature {
     Steer,
     /// Enable collaboration modes (Plan, Code, Pair Programming, Execute).
     CollaborationModes,
+    /// Enable personality selection in the TUI.
+    Personality,
     /// Use the Responses API WebSocket transport for OpenAI by default.
     ResponsesWebsockets,
 }
@@ -543,6 +545,16 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::Personality,
+        key: "personality",
+        stage: Stage::Experimental {
+            name: "Personality",
+            menu_description: "Choose a communication style for Codex.",
+            announcement: "NEW! Update codex's communication style with /personality. Enable in /experimental!",
+        },
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::ResponsesWebsockets,
         key: "responses_websockets",