Fixes gt-9nf: gt spawn should create fresh polecat worktree, never reuse
Changes:
- Add Recreate() method to polecat manager that removes and recreates worktrees
- Modify spawn.go to always recreate existing polecats with fresh worktrees
- Preserves safety checks: blocks if polecat is working or has uncommitted work
- Use --force to bypass uncommitted work checks
This ensures polecats always start with the latest code from the base branch,
avoiding stale code, stale beads, and git history pollution.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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