Promote js_repl to experimental with Node requirement (#12712)

## Summary

- Promote `js_repl` to an experimental feature that users can enable
from `/experimental`.
- Add `js_repl` experimental metadata, including the Node prerequisite
and activation guidance.
- Add regression coverage for the feature metadata and the
`/experimental` popup.

## What Changed

- Changed `Feature::JsRepl` from `Stage::UnderDevelopment` to
`Stage::Experimental`.
- Added experimental metadata for `js_repl` in `core/src/features.rs`:
  - name: `JavaScript REPL`
- description: calls out interactive website debugging, inline
JavaScript execution, and the required Node version (`>= v24.13.1`)
- announcement: tells users to enable it, then start a new chat or
restart Codex
- Added a core unit test that verifies:
  - `js_repl` is experimental
  - `js_repl` is disabled by default
- the hardcoded Node version in the description matches
`node-version.txt`
- Added a TUI test that opens the `/experimental` popup and verifies the
rendered `js_repl` entry includes the Node requirement text.

## Testing

- `just fmt`
- `cargo test -p codex-tui`
- `cargo test -p codex-core` (unit-test phase passed; stopped during the
long `tests/all.rs` integration suite)

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

@@ -23,6 +23,7 @@ use codex_core::config::ConstraintError;
 #[cfg(target_os = "windows")]
 use codex_core::config::types::WindowsSandboxModeToml;
 use codex_core::config_loader::RequirementSource;
+use codex_core::features::FEATURES;
 use codex_core::features::Feature;
 use codex_core::models_manager::manager::ModelsManager;
 use codex_core::skills::model::SkillMetadata;
@@ -5945,6 +5946,30 @@ async fn experimental_features_toggle_saves_on_exit() {
     assert_eq!(updates, vec![(expected_feature, true)]);
 }
 
+#[tokio::test]
+async fn experimental_popup_shows_js_repl_node_requirement() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(None).await;
+
+    let js_repl_description = FEATURES
+        .iter()
+        .find(|spec| spec.id == Feature::JsRepl)
+        .and_then(|spec| spec.stage.experimental_menu_description())
+        .expect("expected js_repl experimental description");
+    let node_requirement = js_repl_description
+        .split(". ")
+        .find(|sentence| sentence.starts_with("Requires Node >= v"))
+        .map(|sentence| sentence.trim_end_matches(" installed."))
+        .expect("expected js_repl description to mention the Node requirement");
+
+    chat.open_experimental_popup();
+
+    let popup = render_bottom_popup(&chat, 120);
+    assert!(
+        popup.contains(node_requirement),
+        "expected js_repl feature description to mention the required Node version, got:\n{popup}"
+    );
+}
+
 #[tokio::test]
 async fn model_selection_popup_snapshot() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(Some("gpt-5-codex")).await;