Bind external events to permanent agent IDs

Notification intake binds each accepted event to a permanent agent ID.
Only that agent sees the event at its next turn boundary. Global retry
keys reject a different target or text, while root receipts without an
identity retain their root binding.

Pending recovery resolves identity before log reservations and does not
redirect work to a replacement with the same name. Delivery, completion,
backup, and import guards preserve that binding without changing saved
envelopes or adding a rename operation.

Issue: 44d8703