Queue external data for the next root turn

Root events stay in the durable agent queue until a turn boundary, so
accepted data cannot enter an active tool chain or its memory view.
Source and event IDs bind retries to the accepted text across restart
and completion. A fixed receipt cap rejects new keys without discarding
pending work or retry protection.

External data carries its receipt time and an explicit trust boundary.
Quoted bodies cannot create image attachments, and raw events do not
produce task-worker completion notices in Telegram. Delivery recovery
can repeat model calls and tool effects, not delivery log entries.

Issue: 44d8703