chore(intake): obtain authoritative SHINES intake API contract (go-live dependency)

Summary

SHINES has no intake API today; CRAIG ships against a mock + a proposed spec (string record_id + string ssn). This tracks obtaining the real SHINES CPS-intake contract (endpoint, auth, response shape, error format) and reconciling the mock/spec to it. Blocks go-live. Per plan §External dependencies.

Acceptance criteria

• Authoritative SHINES intake API contract obtained + documented
• ShinesSink + mock reconciled to the real contract

References

ADR-042 + the plan (docs/modules/ROOT/pages/plans/standalone-intake-backend-plugin.adoc). Part of epic &58.