Create a new workspace chat.

This endpoint is used when a user sends their first message from the empty workspace chat state. It creates the chat shell and persists a goal SystemTurn synchronously — the first user message arrives later via the WebSocket (which itself emits a ChatTurnIntent(ChatKey, USER)).

Chat-shell creation is a first-class operation distinct from "advance a chat by one turn": there is no turn to persist here, so we call get_or_create_chat_shell directly rather than constructing a placeholder Turn just to satisfy ChatTurnIntent's shape. Dedup + principal validation still share the same _create_chat_with_dedup codepath as the turn-driven flows.

The goal SystemTurn is persisted in-route (not emitted as a ChatTurnIntent) so the goal row is guaranteed to exist before the user's first WebSocket message can be processed — otherwise the runner's history-fold may run without the goal in build_system_prompt on the first reply.