Git worktree/branch status can now report "behind remote"

The comparison-status formatter gains a head_behind_remote case that renders as being "behind" the named remote, paired with a new explanatory message clarifying that work starts from the checkout's current commit rather than the remote's tip, since the checkout may be behind as of the last fetch. A separate new message caps the untracked files seeded into a remote session, reporting when untracked files exceed the "v0 seed budget."