Create a stateful chat

Create a persistent conversation whose history, context compression and rolling summary are managed server-side, then post turns with POST /v1/chats/{chatId}/messages. Pick exactly one shape: mindId for a 1:1 chat with a single Mind, mindIds for a multi-Mind conversation where a moderator decides which Mind answers each turn, or audienceIds for a Study chat that fans every message out to all Minds in the attached Audiences and returns aggregated output. Use this when you want the server to remember the conversation; use POST /v1/minds/{mindId}/completion when you manage history yourself. Minds must be owned by you, public, or shared with your team; Audiences must be ones you own, belong to, or that are public or shared with your team. Retired field names are rejected with 400 naming the replacement.