Deliver messages between parent and child agents

A parent can queue a user message for its own child through the daemon.
The child's final reply queues a work message for the parent with the
child's name in brackets. Each delivery wakes its recipient, including
one that is busy, and an empty final reply sends nothing.

The durable queue keeps inputs until their turn completes and saves a
child report with its input acknowledgments. Saved log positions prevent
repeat entries after restart. Interrupted turns can repeat model calls
and tools. Queue failures stop the loops, and backups lock the queue
before the agent files. Turn logs name their agent. Tools and prompts
stay unchanged.