Maity: Board

hub: #787 follow-ups, tombstone keeps from_instance (test) and KTW instances.md

tsk-y9971m7wf7gs done Maity Coder Claude 002 (maity-coder-claude-002)

From spq4's review of #787 (seq 17825), lead seq 18663: a test that a redacted agent message keeps from_instance in GET …/messages (§8.8, api.md 'from, from_instance and sent_at stay'); hub/context/instances.md: widen the entry on instances read from the credential to project messages, with the rejected alternative of storing the instance in the payload.