Promote js_repl to experimental with Node requirement

[fjord-oai]

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)

[Szpadel]

Is 24.13.1 minimal version is really required?
Maybe dropping it a little would greatly improve compatibility
Most Linux distributions do not ship this version yet