USAi catalog (later): reduce sync-usai-models.mjs to build-catalog + emitter

Part of epic #357 (Later — the Phase B cutover). Replace the ~900-line sync-usai-models.mjs monolith with a thin wrapper over build-catalog + the opencode emitter. Safe ONLY because the byte-exact round-trip test (#360) proves no field is lost. The regeneration diff MUST be human-reviewed, not auto-committed (live feeds can re-derive different prices). Deps: #360 green. Live-test: yes.