docs: Document the driver prompt at agent RAG init and the duckdb two-file pairing
Agent startup and graph rag nodes now offer the storage driver choice, so the
RAG page no longer claims it is only asked while creating a RAG with .rag.
Also documents that a duckdb RAG is two files that must travel together, what
Coyote does when only the .yaml arrives, and that a local Qdrant is reached
without going through an ambient proxy.
docs: Document RAG storage drivers, Qdrant attach, and sandbox wiring
Covers the pluggable RAG driver work: the yaml/duckdb/qdrant drivers and how
to choose between them, attaching a read-only remote Qdrant collection with
.rag attach, the driver field on graph rag nodes, and how an attached RAG's
host and API key reach a sandbox.
Also corrects two statements the driver work invalidated: embeddings are no
longer necessarily in-memory, and .rebuild rag now re-embeds the whole corpus
rather than just accommodating document changes.
Dark-Alex-17