feat(rag): docs ingestion + UI/bus enhancements (P3 continuation)

docs: add local-node<->HF-Space connection proof (screenshots + make_proof.py)

fix: 0 test failures; FileService; real RagService; emergency probe; chat return

feat: impl_ref gaps closed — Nemotron/OpenBMB backends, spec refs, extended UI, HOWTO, multi-node E2E