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/core/src/features.rs

@@ -18,12 +18,33 @@ pub(crate) use legacy::LegacyFeatureToggles;
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum Stage {
     Experimental,
-    Beta,
+    Beta {
+        menu_description: &'static str,
+        announcement: &'static str,
+    },
     Stable,
     Deprecated,
     Removed,
 }
 
+impl Stage {
+    pub fn beta_menu_description(self) -> Option<&'static str> {
+        match self {
+            Stage::Beta {
+                menu_description, ..
+            } => Some(menu_description),
+            _ => None,
+        }
+    }
+
+    pub fn beta_announcement(self) -> Option<&'static str> {
+        match self {
+            Stage::Beta { announcement, .. } => Some(announcement),
+            _ => None,
+        }
+    }
+}
+
 /// Unique features toggled via configuration.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum Feature {
@@ -292,13 +313,32 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
-    // Unstable features.
+    FeatureSpec {
+        id: Feature::WebSearchRequest,
+        key: "web_search_request",
+        stage: Stage::Stable,
+        default_enabled: false,
+    },
+    // Beta program. Rendered in the `/experimental` menu for users.
+    FeatureSpec {
+        id: Feature::Skills,
+        key: "skills",
+        stage: Stage::Beta {
+            menu_description: "Define new `skills` for the model",
+            announcement: "NEW! Try the new `skills` features. Enable in /experimental!",
+        },
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::UnifiedExec,
         key: "unified_exec",
-        stage: Stage::Experimental,
+        stage: Stage::Beta {
+            menu_description: "Run long-running terminal commands in the background.",
+            announcement: "NEW! Try Background terminals for long running processes. Enable in /experimental!",
+        },
         default_enabled: false,
     },
+    // Unstable features.
     FeatureSpec {
         id: Feature::RmcpClient,
         key: "rmcp_client",
@@ -308,13 +348,7 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::ApplyPatchFreeform,
         key: "apply_patch_freeform",
-        stage: Stage::Beta,
-        default_enabled: false,
-    },
-    FeatureSpec {
-        id: Feature::WebSearchRequest,
-        key: "web_search_request",
-        stage: Stage::Stable,
+        stage: Stage::Experimental,
         default_enabled: false,
     },
     FeatureSpec {
@@ -347,12 +381,6 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Experimental,
         default_enabled: false,
     },
-    FeatureSpec {
-        id: Feature::Skills,
-        key: "skills",
-        stage: Stage::Experimental,
-        default_enabled: false,
-    },
     FeatureSpec {
         id: Feature::ShellSnapshot,
         key: "shell_snapshot",