feat: 挑战的题目要求发给 AI 当对照清单，学生没提到的不替他做，在“想一想”里问

学生不用再整段贴题目；AI 只做学生说了的，从没提到的要求里挑一项问他打算怎么做。
整段贴了题目的照做。自由创作的题目是学生自己写的，不发。输入框提示改成“用你自己的话说说”。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/ws.ts

@@ -3,6 +3,7 @@ import { all, get, run, iso, uuidOut } from "./db"
 import { userFromAccessToken, type User } from "./auth"
 import {
   canAccessTask,
+  challengeBrief,
   codeAsMarkdown,
   getOrCreateActiveConversation,
   guidanceContext,
@@ -132,7 +133,8 @@ async function handlePrompt(ws: WS, data: any) {
     }))
     let full: string
     try {
-      full = await streamToClient(ws, streamChat(history, model, ws.data.abort.signal))
+      const brief = challengeBrief(ws.data.taskId)
+      full = await streamToClient(ws, streamChat(history, model, ws.data.abort.signal, brief))
     } catch (e) {
       // 客户端自己断开导致的中止不算出错
       if (!ws.data.closed) send(ws, { type: "error", content: userFacingError(e) })