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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Summary

- Display the full branch name in the DocumentationAside panel for
thread branch picker entries, so users can see truncated branch names in
full
- Add a separator between the branch name and descriptive text, matching
the pattern used by the model selector's DocumentationAside

Release Notes:

- Agent: Improved branch picker by displaying the full branch name in
the documentation aside.

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>