Polish Assistant Query Api Discovery

The assistant feedback shows the query layer is now powerful but still hard to self-discover: a capable client can use range filters, search filters, aggregations, blobs, and changes_since, but too much of the correct shape is learned by trial-and-error.

The current implementation already exposes most primitives the assistant asked for:

The reference RS SHALL expose a public schema/capability discovery surface that lets a bearer enumerate the queryable sources and streams visible to that bearer without relying on out-of-band connector IDs or prior stream knowledge.