Changed polecat branch model to use unique timestamped branches
(polecat/<name>-<timestamp>) instead of reusing persistent branches.
This prevents JSONL drift issues where stale polecat branches don't
have recently created beads.
Changes:
- Add(): create unique branch, simplified (no reuse logic)
- Recreate(): create fresh branch, old ones left for GC
- loadFromBeads(): read actual branch from git worktree
- CleanupStaleBranches(): remove orphaned polecat branches
- ListBranches(pattern): new git helper for branch enumeration
- gt polecat gc: new command to clean up stale branches
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>