Expand First Party Parent Child Relations

1. Audit

• Scan first-party manifests for declared relationships and classify each as parent-to-child, reverse/belongs-to, stale, or unsupported.
• Record the classification in design.md or a change-local design note.

2. Manifest Backfill

• Enable only safe parent-to-child query.expand declarations.
• Do not enable reverse relations such as message-to-thread/channel/user or transaction-to-account/category.
• Keep Gmail existing expansions intact.

3. Tests

• Add manifest-level validation that every first-party query.expand entry has a matching relationship and child foreign key.
• Add first-party synthetic record tests for each newly enabled relation.
• Test list and detail expansion.
• Test child grant projection and missing-child-grant rejection.
• Test expand_limit for has-many relations.

4. Docs

• Add one copy-pasteable expand[] example for a first-party stream.
• Document that reverse/belongs-to relations are intentionally deferred.

5. Validation

• Run pnpm --dir reference-implementation exec node --test test/query-contract.test.js.
• Run relevant connector manifest/parser tests if manifests change.
• Run pnpm --dir reference-implementation run verify.
• Run openspec validate expand-first-party-parent-child-relations --strict.
• Run openspec validate --all --strict.