Polish Assistant Query Api Discovery

1. Discovery Endpoint

• Add contract/schema for GET /v1/schema.
• Implement the RS route by reusing/extracting the current stream metadata capability builders.
• Owner-token polyfill mode: enumerate owner-visible connectors and streams without requiring connector_id.
• Client-token mode: scope discovery to the grant's source and streams.
• Add tests for owner discovery, client-grant discovery, field-limited grants, and empty connectors.

2. Query Docs And Cookbook

• Document stream-scoped lexical and semantic search filters with streams[]=...&filter[field]....
• Document changes_since=beginning as the initial incremental-sync path. (Already in cookbook; cross-referenced from new discovery section.)
• Document GET /v1/streams/:stream/aggregate with valid metric, field, group_by, filter[...], and error examples. (Already in cookbook; discovery section nudges toward field_capabilities for valid fields.)
• Document blob byte access through blob_ref.fetch_url and explicitly avoid attachment-specific content endpoint claims.
• Update generated docs/OpenAPI after contract changes.

3. Error Polish

• Audit the assistant-observed 404/400 paths and improve valid-endpoint invalid-parameter messages where safe. (Search/semantic-search filter[...] error now names the supported streams[] shape and rejects filter[stream] / filter[connector_id] explicitly.)
• Keep absent endpoints absent; do not add compatibility aliases unless the design explicitly approves them.

4. Validation

• Run pnpm --dir reference-implementation exec node --test test/query-contract.test.js.
• Run pnpm --filter @pdpp/reference-contract run check:generated.
• Run pnpm --dir reference-implementation run verify.
• Run openspec validate polish-assistant-query-api-discovery --strict.
• Run openspec validate --all --strict.