Record the subagent name in the usage records

Record the subagent name in the usage records

A model request names the subagent that makes it, and the usage record
of the call holds that name in the agent field. A call of the agent
itself has no such field. The name goes only to the usage file, never to
the endpoint, so the usage records show the cost of each subagent in the
shared usage file of the agent.

Issue: b1ac698