Delete a voice agent or twin

Cascade-delete an agent or twin: its persona, identity, knowledge and parent settings row all go with it (voice's own cascade, not something this proxy orchestrates). Requires the voice:write scope explicitly - see require_voice_agent_delete above for why this is self-gated rather than driving a new voice:delete scope.

org_id is pinned server-side; voice filters by (org_id, agent_id) so a cross-org agent_id 404s the same as GET/PUT.