Implements Phase 4 of the two-level beads architecture migration:
- Add hq- prefix helper functions for town-level agent beads
- Add CreateWithID method for deterministic bead creation
- Create gt migrate-agents command with dry-run/execute modes
- Migrate gt-mayor/gt-deacon to hq-mayor/hq-deacon in town beads
- Migrate role beads (gt-*-role) to town beads (hq-*-role)
- Add migration labels to old beads for tracking
- Idempotent: skips already-migrated beads
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
