Add `bd migrate --to-dolt` and `bd migrate --to-sqlite` commands for
migrating between SQLite and Dolt storage backends.
Features:
- `--to-dolt`: Migrate from SQLite to Dolt backend
- Creates backup of SQLite database before migration
- Imports all issues, labels, and dependencies
- Updates metadata.json to use Dolt backend
- Preserves JSONL export configuration
- `--to-sqlite`: Escape hatch to migrate back to SQLite
- Exports all data from Dolt to new SQLite database
- Updates metadata.json to use SQLite backend
Both commands support:
- `--dry-run` flag to preview changes
- `--yes` flag for automated/scripted usage
- `--json` flag for machine-readable output
- Non-cgo stub for builds without CGO support
This implements Part 7 (Migration Tooling) of DOLT-STORAGE-DESIGN.md.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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