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/cli/src/main.rs

@@ -410,7 +410,7 @@ fn stage_str(stage: codex_core::features::Stage) -> &'static str {
     use codex_core::features::Stage;
     match stage {
         Stage::Experimental => "experimental",
-        Stage::Beta => "beta",
+        Stage::Beta { .. } => "beta",
         Stage::Stable => "stable",
         Stage::Deprecated => "deprecated",
         Stage::Removed => "removed",