Add `gt done` command as a convenience wrapper for polecats to signal
their work is ready for the merge queue. This addresses the missing
command referenced in polecat docs.
The command:
- Submits current branch to merge queue (same as gt mq submit)
- Auto-detects issue ID from branch name (polecat/<worker>/<issue>)
- Auto-detects integration branch if source issue has epic parent
- Provides polecat-friendly output messaging
Closes gt-qna4.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
