refactor(db+cli): drop versions_json and pipeline tables; disable legacy commands
Migration:
- Add 027_migrate_mirror_mode.sql
- Drop packages.versions_json (versions now fetched from upstream p2)
- Drop sync_runs, builds, metadata_changes, monthly_installs, status_checks, status_check_changes
Packages layer:
- Remove versions_json references from UpsertPackage, UpsertShellPackage, BatchUpsertPackages, BatchUpsertShellPackages, GetPackagesNeedingUpdate
- Make AllocateSyncRunID/FinishSyncRun no-ops (mirror mode has no sync_runs table)
CLI:
- Disable discover, update, build, deploy, pipeline, check-status commands
- All return "not available in mirror mode" to keep scripts from breaking