Prerequisite for RFC 001 (plugin spec, #743). Removes every direct
`import chromadb` outside the ChromaDB backend itself so the core
modules depend only on the backend abstraction layer.
Extends ChromaBackend with make_client, get_or_create_collection,
delete_collection, create_collection, and backend_version. Adds
update() to the BaseCollection contract. Non-backend callers
(mcp_server, dedup, repair, migrate, cli) now go through the
abstraction; tests patch ChromaBackend instead of chromadb.
With this landed, the RFC 001 spec can be enforced and PalaceStore
(#643) can ship as a plugin without touching core modules.
Igor Lins e Silva