A2A Send Task

Body for POST /a2a/tasks/send - receive a task from an external A2A agent.

Serialised A2A task in responses.

receipt carries the lineage receipt for an inbound task (#2609): the execution evidence a caller verifies offline with bernstein a2a verify --receipt. It is None on read paths, and on write paths when the node could not provision receipt key material - an absent receipt means "unattested", which a caller should treat as unverified rather than trusted.