Previously, `gt done` would fail with "0 commits ahead; nothing to merge"
if work was pushed directly to main instead of via PR. This blocked
polecats from completing even when their work was done, causing them to
become zombies.
Now, if the branch has no commits ahead of main, `gt done` skips MR
creation but still completes successfully - notifying the witness,
cleaning up the worktree, and terminating the session.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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