feat: experimental menu (#8071)

This will automatically render any `Stage::Beta` features.

The change only gets applied to the *next session*. This started as a
bug but actually this is a good thing to prevent out of distribution
push

<img width="986" height="288" alt="Screenshot 2025-12-15 at 15 38 35"
src="https://github.com/user-attachments/assets/78b7a71d-0e43-4828-a118-91c5237909c7"
/>


<img width="509" height="109" alt="Screenshot 2025-12-15 at 17 35 44"
src="https://github.com/user-attachments/assets/6933de52-9b66-4abf-b58b-a5f26d5747e2"
/>

codex-rs/tui/src/app_event.rs

@@ -10,6 +10,7 @@ use codex_protocol::openai_models::ModelPreset;
 use crate::bottom_pane::ApprovalRequest;
 use crate::history_cell::HistoryCell;
 
+use codex_core::features::Feature;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::SandboxPolicy;
 use codex_protocol::openai_models::ReasoningEffort;
@@ -117,6 +118,11 @@ pub(crate) enum AppEvent {
     /// Update the current sandbox policy in the running app and widget.
     UpdateSandboxPolicy(SandboxPolicy),
 
+    /// Update feature flags and persist them to the top-level config.
+    UpdateFeatureFlags {
+        updates: Vec<(Feature, bool)>,
+    },
+
     /// Update whether the full access warning prompt has been acknowledged.
     UpdateFullAccessWarningAcknowledged(bool),