Previously, gt done only killed the polecat session when exitType was
COMPLETED. For DEFERRED, ESCALATED, and PHASE_COMPLETE, it would call
os.Exit(0) which only exited the gt process, leaving Claude running.
Now all exit types terminate the polecat session ("done means gone").
Only COMPLETED also nukes the worktree - other statuses preserve the
work in case it needs to be resumed.
Co-authored-by: julianknutsen <julianknutsen@users.noreply.github>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
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