This PR adjust the name of the worktree within the agent panel: we
display `main` if you're in the main worktree and then the worktree name
when in a linked one. Additionally, I also took the opportunity to fix
something that confused me in the general worktree picker: for the list
item that represented the main worktree, we were using the current
checked out branch name for that item's name. I think that's a bit
incorrect, given the _name_ of that worktree isn't the name of the
branch you have active at that moment. For now, I'm using simply `main`
as the worktree name. Lastly, also added a little blue check icon in the
worktree _and branch_ picker to highlight which worktree/branch is
currently active.

| main worktree | linked worktree | main worktree in picker |
|--------|--------|--------|
| <img width="802" height="338" alt="Screenshot 2026-04-07 at 10  32
2@2x"
src="https://github.com/user-attachments/assets/aad993e6-adc3-4c8d-b01a-abb2597182e7"
/> | <img width="804" height="332" alt="Screenshot 2026-04-07 at 10 
32@2x"
src="https://github.com/user-attachments/assets/e59c5b86-8cee-476c-ab7b-32ae87249c1a"
/> | <img width="804" height="856" alt="Screenshot 2026-04-07 at 10 
36@2x"
src="https://github.com/user-attachments/assets/282d89a8-570e-4e62-b703-853bed9b21ff"
/> |

Release Notes:

- Git: Fixed display of the main worktree name in the worktree picker.