Hide branch name for main worktree threads in sidebar (#54067)

<img width="337" height="752" alt="Screenshot 2026-04-16 at 9 31 04 AM"
src="https://github.com/user-attachments/assets/174b1eb2-6a3a-46f1-abdf-006e13b80c7a"
/>

Release Notes:

- Agent sidebar now shows worktree and branch names only for linked
worktrees.
4f6055e015Richard Feldman committed on 4/16/2026, 3:48:35 PM· committed by GitHubparenteb254be
2 files changedLine totals unavailable