[FEATURE] Implement FAISS Vector Store Backend

[RyanNg1403]

Sub-issue: Implement FAISS Vector Store Backend

Linked to Issue #192

• Summary: Add local FAISS backend (faiss-node) with persistent index files.
• Tasks:
  • Backend: src/core/vector_storage/backend/faiss.ts (init/upsert/query/delete/drop; metrics: l2/ip/cosine; index types: Flat/IVF/HNSW; persistence).
  • Config/Factory: add faiss to config.ts and factory.ts.
  • Tests: CRUD + top-k + persistence reload.
  • Docs: docs/vector-stores.md (setup + config).