Create Chat

Create a tripless chat — no trip, no link, no first message.

An optional anchor_place_id records that the chat was opened on a place's detail sheet (docs/PLACE_ANCHORED_CHATS.md). It is NOT a link: the chat has no ConversationTripLink, so it stays a global chat with the global tool surface, and the anchor only adds the place's dossier to the first turn.

An optional proposal_id instead returns the chat that plan proposal is reviewed in, creating it on the first request (docs/PLAN_PROPOSALS.md).