fix: scope observation to current turn

Require assistant natural language after the latest user message so a completed prompt cannot reuse historical replies when queuing the observation snapshot.

tests/unit/agent-session-sync.test.ts

@@ -220,6 +220,45 @@ describe('local Agent session sync', () => {
     expect(pushSessionDataMock).not.toHaveBeenCalled();
   });
 
+  it('does not reuse an older assistant reply when the current turn has no reply', async () => {
+    const base = createProductSnapshot('conversation-1');
+    queueCodingConversationSessionSync({
+      ...base,
+      nodes: [
+        {
+          kind: 'message',
+          id: 'old-user',
+          role: 'user',
+          status: 'complete',
+          blocks: [{ kind: 'text', id: 'old-question', text: '旧问题', status: 'complete' }],
+        },
+        {
+          kind: 'message',
+          id: 'old-assistant',
+          role: 'assistant',
+          status: 'complete',
+          blocks: [{ kind: 'text', id: 'old-answer', text: '旧回答', status: 'complete' }],
+        },
+        {
+          kind: 'message',
+          id: 'current-user',
+          role: 'user',
+          status: 'complete',
+          blocks: [{ kind: 'text', id: 'current-question', text: '新问题', status: 'complete' }],
+        },
+      ],
+      run: {
+        status: 'idle',
+        runId: 'run-2',
+        settledAt: Date.parse('2026-08-24T08:01:00.000Z'),
+        terminalReason: 'completed',
+      },
+    });
+    await flushPendingAgentSessionSync();
+
+    expect(pushSessionDataMock).not.toHaveBeenCalled();
+  });
+
   it('never uploads a pending snapshot under a different authenticated account', async () => {
     queueAgentSessionSync('prj_1', 'ses_1', [
       { id: 'user-1', role: 'user', content: '账号 A 的问题' },