feat: add schema migration utilities for versioned state files
Implements MigrationRegistry for managing versioned JSON file migrations:
- Version tracking for gitlab_bead_map.json, state.json, settings.json
- Sequential migration execution (v1→v2→v3)
- Error handling for future versions, missing versions, failed migrations
- Factory functions for each file type's registry
Includes 12 tests covering migration chains, error cases, and idempotency.
bd-1jf
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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