feat: 对接设计 Agent Gateway WebSocket

需求：服务端统一 Agent Gateway 将设计任务状态流切换为 WebSocket，客户端需要实时展示生成任务并支持断线恢复。

实现：Electron Main 管理 Session、一次性 Ticket、WebSocket 心跳与游标续传，按关闭码回收会话；Renderer 继续通过本机 Host API 的 SSE 投影接收任务事件，并保留 REST 降级同步。

验证：typecheck、变更文件 ESLint、37 个聚焦测试及 build:vite 通过。

tests/unit/image-workspace-route.test.ts

@@ -215,4 +215,60 @@ describe('AI design Main route boundary', () => {
     expect(response.headers.get('content-range')).toBe('bytes 0-6/100');
     expect(Buffer.concat(response.chunks).toString()).toBe('partial');
   });
+
+  it('relays normalized task events over local SSE and forwards the opaque resume cursor', async () => {
+    const close = vi.fn();
+    const openWorkspaceEvents = vi.fn().mockResolvedValue({
+      events: (async function* () {
+        yield {
+          id: 'session-one:7',
+          type: 'design.generation_task.updated' as const,
+          workspaceId: 'workspace/one',
+          workspaceViewRevision: 8,
+          generationTask: {
+            taskId: 'task-live',
+            workspaceId: 'workspace/one',
+            medium: 'image' as const,
+            status: 'succeeded' as const,
+            briefVersion: 1,
+            briefSummary: '海洋公益海报',
+            quoteId: 'quote-live',
+            quotedDesignPoints: 1,
+            failureCode: null,
+            resultAssets: [],
+            createdAt: '2026-08-02T10:00:00Z',
+            updatedAt: '2026-08-02T10:02:00Z',
+          },
+        };
+      })(),
+      close,
+    });
+    const request = createRequest('GET');
+    request.headers = { 'last-event-id': 'session-one:6' };
+    const response = new MediaResponse();
+
+    const handled = await handleImageWorkspaceRoutes(
+      request,
+      response as unknown as ServerResponse,
+      new URL(
+        'http://127.0.0.1/api/works/image-workspace/workspaces/workspace%2Fone/events',
+      ),
+      { imageWorkspace: { openWorkspaceEvents } } as unknown as HostApiContext,
+    );
+
+    expect(handled).toBe(true);
+    expect(openWorkspaceEvents).toHaveBeenCalledWith({
+      workspaceId: 'workspace/one',
+      afterEventId: 'session-one:6',
+    });
+    expect(response.statusCode).toBe(200);
+    expect(response.headers.get('content-type')).toBe('text/event-stream; charset=utf-8');
+    expect(response.headers.get('x-accel-buffering')).toBe('no');
+    expect(Buffer.concat(response.chunks).toString()).toContain([
+      'id: session-one:7',
+      'event: design.generation_task.updated',
+      'data: {"id":"session-one:7"',
+    ].join('\n'));
+    expect(close).toHaveBeenCalledOnce();
+  });
 });