Polish Reference Api Discovery Seams

1. Discovery Index

• Add a contract entry for GET / (RS-side, unauthenticated) returning a small JSON pointer.
• Add a contract entry for GET / (AS-side, unauthenticated) returning a small JSON pointer.
• Implement the route in buildAsApp and buildRsApp ahead of any auth middleware.
• Reuse resolveReferenceRevision so the index always reports the same revision the response header uses.
• Add tests that cover: AS index shape, RS index shape, and that the index works without a bearer token.

2. Well-Known Discovery Hints

• Extend buildProtectedResourceMetadata to accept and emit a pdpp_discovery_hints block.
• Build the block from the same runtime state used for capabilities; expose: schema_endpoint, query_base, search.{endpoint, scope_param, filter_requires_single_stream}, aggregate.endpoint_template, changes_since_bootstrap, blob_indirection, and hybrid_pagination_supported when hybrid is advertised.
• Update the contract response schema for getProtectedResourceMetadata to permit the new block.
• Add a test that asserts the hints block is present on the canonical RS startup, and that hybrid_pagination_supported is omitted when hybrid is not advertised.

3. Reference Revision Operational Path

• Add ARG PDPP_REFERENCE_REVISION and an ENV PDPP_REFERENCE_REVISION line in the reference Dockerfile stage so the build can bake a revision into the image.
• Confirm resolveReferenceRevision honors PDPP_REFERENCE_REVISION (already implemented) and add a test that asserts the env override is reflected in the response header and the discovery-index body.
• Document the build-arg path in the reference runbook (or the closest existing operator-facing doc).

4. Deferred Slice Capture

• Write design-notes/connector-scoping-and-group-by.md capturing the deferred work, the affected specs, and the test scenarios a follow-up change should add.

5. Validation

• Run pnpm --dir reference-implementation exec node --test test/provider-metadata.test.js test/query-contract.test.js.
• Run pnpm --filter @pdpp/reference-contract run check:generated (and verify if generated docs change).
• Run pnpm --dir reference-implementation run verify.
• Run openspec validate polish-reference-api-discovery-seams --strict.
• Run openspec validate --all --strict.