feat(rag): ingest CLI (markdown/PDF → chunks → FAISS) + sample KB fixtures

fix(rag/store): copy vectors before in-place normalize_L2 (no caller mutation)

feat(rag): FAISS inner-product store with chunk metadata + roundtrip

feat(rag): fastembed wrapper (Embedder, bge-small-en-v1.5, 384-dim)

feat(rag): paragraph-aware chunker (chunk_text)