Delete Chat Agent

Delete a specialist chat agent (superadmin only).

Refuses a group head or the tenant's default with 409 — those must be re-pointed first. Otherwise the agent is pruned from any group it belongs to (each re-provisioned) and deleted.

Destructive cascade: Conversation.chat_agent_id and ConversationMessage.conversation_id are ON DELETE CASCADE, so deleting a chat agent permanently removes every conversation and message held against it. The cockpit must confirm this with the operator before calling this route.