For agents, on top of README.md, which they read first.

• A call that got no response keeps nothing. There is nothing to return next time, so that request may be sent again. Do not store errors.
• sent_now is true for one ask only: the one that put the request on the wire. An ask that waited on someone else's call gets Record::kept.
• These types are sent as JSON between a Worker and an object that may be on different code for some seconds after a deploy. A renamed field or variant fails those asks until both are on the new code.