Continue a Mac-synced chat in the cloud

Continue a chat synced from a Mac (the agent columnId names, the first when omitted) in the cloud, from the transcript the platform already holds, with text (and optional files) as the new session's first message, as a send on the web does. It stays the same chat: the new session's history starts with the synced conversation and playgroundId is this thread. The chat's owner and any workspace member who may use it can continue it; the new session is the caller's. Works with the Mac offline. 409 continue_pending means ask again with the same requestId; 422 not_mirrored when the chat did not come from a Mac.