agent_ui: Clarify multi-root agent warning text (#57874)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A
bedfe32fdcBen Brandt committed on 5/28/2026, 8:19:11 AM· committed by GitHubparent58f84cf
1 file changedLine totals unavailable