metadata.task_description is not in the consumer snapshot — the trigger
consumer only populates metadata.automation from the outbox typed_payload.
artifacts.item_body IS populated via dispatch_service from
item_snapshot["body"] (set from typed_payload.body in the consumer,
which comes from item.get("body") in emit_work_item_event).
For M39 items the body contains the full task description, so
${artifacts.item_body} gives the agent complete implementation instructions.
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