Delete agent

**Permanently deletes the agent and all of its data.** This is a hard delete, not an archive — there is no recovery via the public API. Specifically, the call removes: the agent row; the agent's saved record on the upstream provider (VAPI/Retell/Inspra); the agent's calendar config and Google Calendar (if any); knowledge document links; prompt version history; appointments; and **all conversations (call records, transcripts, recordings, analysis) tied to this agent**. Leads owned by the agent are also deleted. Campaigns that referenced the agent survive but are orphaned (their agent_id is set to null). Returns **409 conflict** if the agent has campaigns in scheduled, active, or paused status — stop those first. A second delete after success returns 404 because the row is gone.