feat: Tidy up git integration

[juliusmarminge]

Currently, when in "Local" mode, the git branch will be the one you're checked out on, not the branch that the thread used without any indication. this is confusing if you checkout some other branch then come back to the thread to send some follow up and it gets sent on a different branch

should be:

• store gitBranch in the thread

• if mode === "local" && currentlyCheckedOutBranch !== thread.gitBranch, show an indicator of some form

• before sending the next message, try checking out that branch? with helper actions to stash etc if there are changes

• thread.gitBranch should be mutable, as i might start a thread on main then checkout a feature branch.