This PR adds some adjustments to the thread item component so metadata
labels (project, worktree, and branch names) properly show up and
truncate in different places, as well as fixes the thread switcher by
making hover auto-select the item, making it more consistent with the
regular tab switcher. We still have to figure out how to make the modal
dismiss on click, though; that's pending.

Ended up also cleaning up the thread item's tooltip a bit, and tweaking
the preview examples we have for it.

Release Notes:

- Agent: Fixed worktree and branch labels not showing up in the thread
switcher.
- Agent: Fixed the thread switcher not selecting on hover.