Aligned feature stage names with public feature maturity stages (#9929)

We've recently standardized a [feature maturity
model](https://developers.openai.com/codex/feature-maturity) that we're
using in our docs and support forums to communicate expectations to
users. This PR updates the internal stage names and descriptions to
match.

This change involves a simple internal rename and updates to a few
user-visible strings. No functional change.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__experimental_features_popup.snap

@@ -3,7 +3,7 @@ source: tui/src/chatwidget/tests.rs
 expression: popup
 ---
   Experimental features
-  Toggle beta features. Changes are saved to config.toml.
+  Toggle experimental features. Changes are saved to config.toml.
 
 › [ ] Ghost snapshots  Capture undo snapshots each turn.
   [x] Shell tool       Allow the model to run shell commands.

codex-rs/tui/src/chatwidget/tests.rs

@@ -2911,13 +2911,13 @@ async fn experimental_features_popup_snapshot() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(None).await;
 
     let features = vec![
-        BetaFeatureItem {
+        ExperimentalFeatureItem {
             feature: Feature::GhostCommit,
             name: "Ghost snapshots".to_string(),
             description: "Capture undo snapshots each turn.".to_string(),
             enabled: false,
         },
-        BetaFeatureItem {
+        ExperimentalFeatureItem {
             feature: Feature::ShellTool,
             name: "Shell tool".to_string(),
             description: "Allow the model to run shell commands.".to_string(),
@@ -2937,7 +2937,7 @@ async fn experimental_features_toggle_saves_on_exit() {
 
     let expected_feature = Feature::GhostCommit;
     let view = ExperimentalFeaturesView::new(
-        vec![BetaFeatureItem {
+        vec![ExperimentalFeatureItem {
             feature: expected_feature,
             name: "Ghost snapshots".to_string(),
             description: "Capture undo snapshots each turn.".to_string(),