feat(agent): 前端补齐 P0(AskUser 问答 UI + 停止按钮)

- AskModal: agent 调用 AskUserQuestion 时弹出问答框(多选 options +
  自由文本),提交走 POST /api/agent/answer;ask/ask_resolved SSE
  事件接入 handleEvent。
- 停止按钮: AbortController 绑定 chat fetch,abort 即 drop SSE
  stream → 后端 CancelGuard 触发 CancellationToken 取消 agent loop;
  区分 [已停止] 与 [连接中断]。

src/web/pages/AgentWorkbench.tsx

@@ -31,6 +31,12 @@ type ApprovalPrompt = {
   risk: string;
 };
 
+type AskPrompt = {
+  requestId: string;
+  question: string;
+  options: string[];
+};
+
 type SessionUsage = {
   inputTokens: number;
   outputTokens: number;
@@ -88,11 +94,16 @@ export default function AgentWorkbench() {
   const [sessionId, setSessionId] = useState<string | null>(null);
   const [usage, setUsage] = useState<SessionUsage | null>(null);
   const [approval, setApproval] = useState<ApprovalPrompt | null>(null);
+  const [ask, setAsk] = useState<AskPrompt | null>(null);
+
+  // Lets the user stop a running turn: aborting the fetch drops the SSE stream,
+  // which the backend observes and uses to cancel the agent loop.
+  const abortRef = useRef<AbortController | null>(null);
 
   const scrollRef = useRef<HTMLDivElement | null>(null);
   useEffect(() => {
     scrollRef.current?.scrollTo({ top: scrollRef.current.scrollHeight, behavior: 'smooth' });
-  }, [messages, approval]);
+  }, [messages, approval, ask]);
 
   const authToken = useCallback(
     () => getSessionToken(audience, localStorage) ?? '',
@@ -132,6 +143,25 @@ export default function AgentWorkbench() {
     [authToken, toast],
   );
 
+  const submitAnswer = useCallback(
+    async (requestId: string, answer: string) => {
+      setAsk(null);
+      try {
+        await fetch('/api/agent/answer', {
+          method: 'POST',
+          headers: {
+            'Content-Type': 'application/json',
+            Authorization: `Bearer ${authToken()}`,
+          },
+          body: JSON.stringify({ request_id: requestId, answer }),
+        });
+      } catch {
+        toast.error('提交回答失败');
+      }
+    },
+    [authToken, toast],
+  );
+
   const handleEvent = useCallback(
     (e: SseEvent) => {
       const d = e.data ?? {};
@@ -179,6 +209,19 @@ export default function AgentWorkbench() {
             risk: String(d.risk ?? ''),
           });
           break;
+        case 'ask':
+          setAsk({
+            requestId: String(d.request_id ?? ''),
+            question: String(d.question ?? ''),
+            options: Array.isArray(d.options)
+              ? (d.options as unknown[]).map((o) => String(o))
+              : [],
+          });
+          break;
+        case 'ask_resolved':
+          // Agent moved on (answered elsewhere, or timed out) — drop the modal.
+          setAsk((cur) => (cur && cur.requestId === d.request_id ? null : cur));
+          break;
         case 'result':
           setUsage({
             inputTokens: Number(d.input_tokens ?? 0),
@@ -212,6 +255,7 @@ export default function AgentWorkbench() {
 
     setRunning(true);
     setApproval(null);
+    setAsk(null);
     setMessages((prev) => [
       ...prev,
       { role: 'user', text: prompt, tools: [] },
@@ -219,6 +263,8 @@ export default function AgentWorkbench() {
     ]);
     setInput('');
 
+    const controller = new AbortController();
+    abortRef.current = controller;
     try {
       const response = await fetch('/api/agent/chat', {
         method: 'POST',
@@ -231,6 +277,7 @@ export default function AgentWorkbench() {
           session_id: sessionId,
           permission_mode: mode,
         }),
+        signal: controller.signal,
       });
       if (!response.ok) {
         const detail = await response.text().catch(() => '');
@@ -238,22 +285,36 @@ export default function AgentWorkbench() {
       }
       await readSse(response, handleEvent);
     } catch (err) {
+      if (controller.signal.aborted) {
+        // User pressed stop — not an error.
+        updateAssistant((m) => {
+          m.text += '\n\n[已停止]';
+        });
+      } else {
         toast.error(`请求失败: ${err instanceof Error ? err.message : String(err)}`);
         updateAssistant((m) => {
           m.text += '\n\n[连接中断]';
         });
+      }
     } finally {
+      abortRef.current = null;
       setRunning(false);
       setApproval(null);
+      setAsk(null);
     }
   }, [authToken, handleEvent, input, mode, running, sessionId, toast, updateAssistant]);
 
+  const stop = useCallback(() => {
+    abortRef.current?.abort();
+  }, []);
+
   const newSession = useCallback(() => {
     if (running) return;
     setMessages([]);
     setSessionId(null);
     setUsage(null);
     setApproval(null);
+    setAsk(null);
   }, [running]);
   return (
     <div style={{ display: 'flex', flexDirection: 'column', height: '100%', gap: 12, padding: 16 }}>
@@ -340,6 +401,14 @@ export default function AgentWorkbench() {
         />
       )}
 
+      {/* Ask-user-question modal */}
+      {ask && (
+        <AskModal
+          prompt={ask}
+          onSubmit={(answer) => submitAnswer(ask.requestId, answer)}
+        />
+      )}
+
       {/* Composer */}
       <div style={{ display: 'flex', gap: 8 }}>
         <textarea
@@ -370,6 +439,21 @@ export default function AgentWorkbench() {
         >
           {running ? '运行中…' : '发送'}
         </button>
+        {running && (
+          <button
+            onClick={stop}
+            style={{
+              padding: '0 16px',
+              borderRadius: 8,
+              background: '#ef4444',
+              color: '#fff',
+              border: 'none',
+              cursor: 'pointer',
+            }}
+          >
+            停止
+          </button>
+        )}
       </div>
     </div>
   );
@@ -480,3 +564,106 @@ function ApprovalModal({
     </div>
   );
 }
+
+function AskModal({
+  prompt,
+  onSubmit,
+}: {
+  prompt: AskPrompt;
+  onSubmit: (answer: string) => void;
+}) {
+  const [selected, setSelected] = useState<string[]>([]);
+  const [freeText, setFreeText] = useState('');
+
+  const toggle = (opt: string) => {
+    setSelected((cur) =>
+      cur.includes(opt) ? cur.filter((o) => o !== opt) : [...cur, opt],
+    );
+  };
+
+  // Combine chosen options and any free-form text into one reply.
+  const compose = (): string => {
+    const parts: string[] = [];
+    if (selected.length > 0) parts.push(selected.join(', '));
+    const extra = freeText.trim();
+    if (extra) parts.push(extra);
+    return parts.join('\n');
+  };
+
+  const answer = compose();
+  const canSubmit = answer.length > 0;
+
+  return (
+    <div
+      style={{
+        position: 'fixed',
+        inset: 0,
+        background: 'rgba(0,0,0,0.5)',
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'center',
+        zIndex: 1000,
+      }}
+    >
+      <div
+        style={{
+          background: 'var(--bg, #1a1a1a)',
+          border: '1px solid var(--border-color, #333)',
+          borderRadius: 12,
+          padding: 20,
+          width: 'min(520px, 92vw)',
+          maxHeight: '80vh',
+          overflowY: 'auto',
+        }}
+      >
+        <h3 style={{ margin: '0 0 8px', fontSize: 16 }}>Agent 提问</h3>
+        <p style={{ margin: '0 0 12px', fontSize: 14, lineHeight: 1.6, whiteSpace: 'pre-wrap' }}>
+          {prompt.question}
+        </p>
+        {prompt.options.length > 0 && (
+          <div style={{ display: 'flex', flexDirection: 'column', gap: 6, marginBottom: 12 }}>
+            {prompt.options.map((opt) => (
+              <label key={opt} style={{ display: 'flex', alignItems: 'center', gap: 8, fontSize: 13, cursor: 'pointer' }}>
+                <input
+                  type="checkbox"
+                  checked={selected.includes(opt)}
+                  onChange={() => toggle(opt)}
+                />
+                <span>{opt}</span>
+              </label>
+            ))}
+          </div>
+        )}
+        <textarea
+          value={freeText}
+          onChange={(e) => setFreeText(e.target.value)}
+          onKeyDown={(e) => {
+            if (e.key === 'Enter' && (e.metaKey || e.ctrlKey) && canSubmit) {
+              e.preventDefault();
+              onSubmit(answer);
+            }
+          }}
+          placeholder={prompt.options.length > 0 ? '补充说明（可选）…' : '输入你的回答…'}
+          rows={3}
+          style={{ width: '100%', boxSizing: 'border-box', padding: 10, borderRadius: 8, resize: 'vertical', fontFamily: 'inherit', fontSize: 14 }}
+        />
+        <div style={{ display: 'flex', justifyContent: 'flex-end', gap: 8, marginTop: 16 }}>
+          <button
+            onClick={() => onSubmit(answer)}
+            disabled={!canSubmit}
+            style={{
+              padding: '8px 16px',
+              borderRadius: 8,
+              background: canSubmit ? 'var(--primary, #4a9eff)' : 'var(--border-color, #333)',
+              color: '#fff',
+              border: 'none',
+              cursor: canSubmit ? 'pointer' : 'default',
+            }}
+          >
+            提交回答
+          </button>
+        </div>
+      </div>
+    </div>
+  );
+}