jgs-v1-migrate-write

v1→v2 migration write phase: build the v2 model from the live v1 model (packages, part definitions/usages, and requirement definitions/usages, with v1 requirement IDs carried over as join keys). PRO tier required; write-tier on the jgs-magic-sysmlv2-mcp bridge. Requires jgs-magic-sysmlv1-mcp AND jgs-magic-sysmlv2-mcp both running. Trigger: "write the migration", "build v2 from v1", "execute the migration", "write phase", "migrate v1 to v2 now".