Part of #430.

Ship this first, separately, before any other work in the pivot epic.

Tasks

• Add a dismissible banner to the React app announcing that the web app is being discontinued and pointing at the new direction (linter + recommended stack + skills)
• Link the banner to the tracking issue (Pivot scope42 to an agent-native, format-agnostic workflow #430) so existing users can follow progress
• Release and deploy a final version of the app that carries the banner
• Only after the announcement is live, start the pivot work in the other sub-issues

Notes

This is the only piece of work touching the legacy app before it is deleted. The banner must be visible in normal usage, not hidden behind a settings dialog.