When a polecat runs `gt done` with COMPLETED status, it now nukes its own
worktree before exiting. This is the self-cleaning model - polecats clean
up after themselves, reducing Witness/Deacon cleanup burden.
The self-nuke is:
- Only attempted for polecats (not Mayor/Witness/Deacon/Refinery)
- Only on COMPLETED status (not ESCALATED/DEFERRED)
- Non-fatal: if it fails, Witness will handle cleanup
Closes: gt-fqcst
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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