fix(workflows): use artifacts.item_body not metadata.task_description

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.

task-templates/workflow/airouter-impl-ready@1.yaml

@@ -28,7 +28,7 @@ nodes:
       You are implementing source code for the following task.
 
       ## Task Description
-      ${metadata.task_description}
+      ${artifacts.item_body}
 
       ## Instructions
       1. Read the task description carefully — it specifies the file(s) to create/modify and the exact requirements.