Remove ghost snapshots (#19481)

## Summary
- Remove `ghost_snapshot` / `GhostCommit` from the Responses API surface
and generated SDK/schema artifacts.
- Keep legacy config loading compatible, but make undo a no-op that
reports the feature is unavailable.
- Clean up core history, compaction, telemetry, rollout, and tests to
stop carrying ghost snapshot items.

## Testing
- Unit tests passed for `codex-protocol`, `codex-core` targeted undo and
compaction flows, `codex-rollout`, and `codex-app-server-protocol`.
- Regenerated config and app-server schemas plus Python SDK artifacts
and verified they match the checked-in outputs.

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

@@ -1,11 +1,13 @@
 ---
-source: tui/src/chatwidget/tests.rs
+source: tui/src/chatwidget/tests/popups_and_settings.rs
 expression: popup
 ---
   Experimental features
   Toggle experimental features. Changes are saved to config.toml.
 
-› [ ] Ghost snapshots  Capture undo snapshots each turn.
+› [ ] JavaScript REPL  Enable a persistent Node-backed JavaScript REPL for
+                       interactive website debugging and other inline
+                       JavaScript execution capabilities.
   [x] Shell tool       Allow the model to run shell commands.
 
   Press space to select or enter to save for next conversation

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

@@ -1692,9 +1692,9 @@ async fn experimental_features_popup_snapshot() {
 
     let features = vec![
         ExperimentalFeatureItem {
-            feature: Feature::GhostCommit,
-            name: "Ghost snapshots".to_string(),
-            description: "Capture undo snapshots each turn.".to_string(),
+            feature: Feature::JsRepl,
+            name: "JavaScript REPL".to_string(),
+            description: "Enable a persistent Node-backed JavaScript REPL for interactive website debugging and other inline JavaScript execution capabilities.".to_string(),
             enabled: false,
         },
         ExperimentalFeatureItem {
@@ -1715,12 +1715,12 @@ async fn experimental_features_popup_snapshot() {
 async fn experimental_features_toggle_saves_on_exit() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
 
-    let expected_feature = Feature::GhostCommit;
+    let expected_feature = Feature::JsRepl;
     let view = ExperimentalFeaturesView::new(
         vec![ExperimentalFeatureItem {
             feature: expected_feature,
-            name: "Ghost snapshots".to_string(),
-            description: "Capture undo snapshots each turn.".to_string(),
+            name: "JavaScript REPL".to_string(),
+            description: "Enable a persistent Node-backed JavaScript REPL for interactive website debugging and other inline JavaScript execution capabilities.".to_string(),
             enabled: false,
         }],
         chat.app_event_tx.clone(),