Replace polecat state.json with beads assignee field for state management:
- Remove state.json read/write from polecat.Manager
- Add loadFromBeads() to derive state from issue.assignee field
- Update AssignIssue() to set issue.assignee in beads
- Update ClearIssue() to clear assignee from beads
- Update SetState() to work with beads or gracefully degrade
- Add ListByAssignee and GetAssignedIssue to beads package
- Update spawn to create beads issues for free-form tasks
- Update tests for new beads-based architecture
State derivation:
- Polecat exists: worktree directory exists
- Polecat assigned: issue.assignee = 'rig/polecatName'
- Polecat working: issue.status = open/in_progress
- Polecat done: issue.status = closed or no assignee
Fixes: gt-qp98
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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