chat.yakka.postMessage

Write a message to the author's own repo, under their OAuth grant (Law 4: write admission belongs to the author's PDS, never to the appview). The appview is the caller's agent here, not an authority — a refusal from the PDS is reported as one rather than absorbed. Resolves once the record is ingested and sequenced, by which time the client has already shown a provisional copy and will reconcile on (uri, cid) (ADR 0011).