Deliver routed WhatsApp batches to children

A routed batch enters the durable agent message queue as work and wakes
only its child. Saved log IDs prevent a second entry on restart, and a
receipt remains until inbox removal even if the child finishes first.
Receipts use batch numbers rather than a watermark because chats can
close out of number order.

Unrouted batches keep the root path. A missing child sends new inputs
and its queued external batches to the root. Routed events keep their
WhatsApp source without a subagent completion status, and daemon logs
name the destination without message text or chat IDs.