_sync_target_branch previously did a bare 'git checkout <target>' with no
fallback. In the shared-agent-clone model the leaf developer's workspace
often has only the task branch locally; the parent/cell branch exists only
on origin. That produced a SERVICE_ERROR which cycled the task back to
blocked every time the PM retried complete().
Now, when checkout fails, we fetch the target branch from origin and create a
tracking branch before the pull. Includes regression tests for the local,
fallback, and origin-missing paths.
Renn F
4a18dbe4c8