Get Or Create Ambient Conversation

Flywheel V1.3 — get-or-create the subject's ambient FDE conversation.

Subject-polymorphic via XOR query params: exactly one of voice_agent_id or voice_agent_group_id must be set; passing both or neither returns 400. The ambient conversation is the one where flywheel_run_id IS NULL — distinct from cycle-scoped conversations created by V1.1a's preview_plan. The cockpit reads this on mount in readonly mode (no active cycle); cycle conversations are surfaced via ActiveCycleResponse.conversation_id instead.

Auto-creates if the subject has no prior ambient conversation, so the cockpit always has somewhere to stream into. Idempotent: two sequential calls return the same conversation id (modulo the documented V1.3 race for concurrent first-mount; V1.2 closes that with a partial unique index).

Returns FDEConversationDetailRead with messages populated so the cockpit's first paint already has chat history without a second round-trip.