Delete Agent Definition

Soft delete: the definition keeps its publish pointer and revisions, every live read — resolution included — stops seeing it, and its slug is freed for the living immediately. Active publishable keys pointing here are repointed at the org's default assistant (cleared when this IS the assistant, which also strands keyless serving surfaces), and a Slack agent is disconnected from Slack — so while any of those consumers exists, the delete answers a 409 naming them until the caller repeats it with confirm=true.