Adds MongoMemoryStore with Atlas Vector Search recall alongside the thread store.
Embedding source-agnostic with two first-class paths (no silent fallback):
bring-your-own query vector (default) and Atlas Automated Embedding (auto_embed,
autoEmbed index, recall by query text). episodic/semantic kinds, get_recent recency
recall, prefiltered $vectorSearch, ensure_vector_index, as_save_hook.

25 acceptance tests (mongomock); agentic Gemini demo (demo/memory_demo.py) verified
on Atlas in both embedding modes.