feat(get): Add branch checkout mode

[abhinav]

Allow treehouse get -b <branch> and treehouse get --lease -b <branch>
to create a new local branch after acquisition.
The worktree still enters through the existing detached acquisition path,
then checks out the requested branch at the acquired HEAD.

Preserve existing dirty-exit behavior for plain treehouse get.
Branch mode keeps a dirty declined-cleanup worktree on the branch,
while clean returns still detach and restore the pool state.

[abhinav]

This change is part of the following stack:

• feat(get): Add branch checkout mode #46 ◀

_{Change managed by git-spice.}