Moves the four schema-migration handlers (--migrate-wom,
--migrate-zone, --migrate-project, --migrate-jsondbc) out of
main.cpp into a new cli_migrate.{hpp,cpp} module. Same
behavior — they're idempotent in-place upgraders for older
WOM revisions and JSON DBC sidecar schemas. main.cpp shrinks
by 282 lines (11,261 → 10,979).
