Polish Assistant Query Api Discovery

Why

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.

What Changes

• Add a one-shot schema/capability discovery surface for the reference RS so agents can discover connectors, streams, schemas, filterability, aggregation support, expansion support, blob affordances, and freshness without out-of-band connector IDs.
• Tighten public/reference docs and generated cookbook examples for streams[] search filters, changes_since=beginning, GET /v1/streams/:stream/aggregate, blob_ref.fetch_url, and unsupported endpoint shapes.
• Improve error messages where a caller uses the wrong endpoint or wrong parameter spelling, without broadening the protocol.
• Keep existing per-stream metadata as the source of truth; the new discovery surface aggregates it rather than inventing a second capability model.

Capabilities

New Capabilities

Modified Capabilities

• reference-implementation-architecture: add a reference RS discovery requirement for schema/capability enumeration and self-service query guidance.

Impact

• reference-implementation/server/index.js
• reference-implementation/server/records.js
• reference-implementation/test/query-contract.test.js
• packages/reference-contract/src/public/index.ts
• generated OpenAPI/docs artifacts
• apps/web/content/docs/spec-data-query-api.md
• apps/web/content/docs/spec-core.md