Assistant chats

Read a chat the user had with the assistant inside the Needle web app, to continue it here. When `handoff` is set, the assistant passed the task on: its `brief` holds the goal, the ids involved, what was already found, and what is left. Start from the brief and carry the task out with your Needle tools. Messages hold each turn's text and the tools it called. Calls since the last user message keep their results; earlier ones do not.