Documented the `metadata` field on ingestions

The ingest payload's `metadata` field has always round-tripped from an inbound POST to your app code, but wasn't documented — and the SDK JSDoc previously referenced the wrong field names (`meta`/`receivedAt` instead of the actual `metadata`/`createdAt`). Docs and the system prompt now correctly describe `metadata`, `createdAt`, and a recommended versioning pattern for callers.