Closes https://github.com/zed-industries/zed/issues/53262
- Remove the ability to pick a branch from the agent panel; delegate
this to the title bar picker
- Make the worktree creation earger, just as you selected whether you
want to create it from main or current branch
- Remove flicker when creating a new worktree and switching to a
previously existing one
- Improve some UI stuff: how we display that a worktree is
creating/loading, the branch and worktree icons, etc.
- Fixed a bug where worktrees in a detached HEAD state wouldn't show up
in the worktree pickers
A big part of the diff of this PR is the removal of everything involved
with the `StartThreadIn` enum/the set up involved in only creating the
worktree by the time of the first prompt send.
Release Notes:
- Agent: Improved and simplified the UX of creating threads in Git
worktrees.
- Git: Fixed a bug where worktrees in a detached HEAD state wouldn't
show up in the worktree picker.
---------
Co-authored-by: Nathan Sobo <nathan@zed.dev>
