feat: 新增子 Agent 插件 (sub_agent) - 多 Agent 协作系统

自研协议，主 AI 可以：
- spawn: 创建子 Agent 并分配独立任务
- list/status: 查看所有子 Agent 状态
- result: 获取子 Agent 执行结果
- send: 向运行中的子 Agent 发送消息
- kill/clean: 终止和清理

特性：
- 每个子 Agent 独立对话历史
- 后台线程异步执行
- 自动提取并执行工具调用
- 完成信号检测
- 并行多任务支持

测试: 113 个用例全部通过

tests/test_plugins.py

@@ -353,6 +353,65 @@ class TestBrowserAuto:
         assert len(info["keywords"]) > 0
 
 
+# ══════════════════════════════════════
+#  引擎加载测试
+# ══════════════════════════════════════
+
+class TestSubAgent:
+    """sub_agent 功能测试"""
+
+    @pytest.fixture
+    def sa(self):
+        mod = load_module("sub_agent", os.path.join(PLUGINS_DIR, "sub_agent.py"))
+        return mod.Plugin()
+
+    def test_list_empty(self, sa):
+        result = sa.handle("list")
+        assert "没有" in result
+
+    def test_stats_empty(self, sa):
+        result = sa.handle("stats")
+        assert "总数: 0" in result
+
+    def test_spawn(self, sa):
+        result = sa.handle("spawn tester 测试任务")
+        assert "已创建" in result
+        assert "agent_tester" in result
+
+    def test_spawn_and_list(self, sa):
+        sa.handle("spawn a 任务A")
+        sa.handle("spawn b 任务B")
+        import time; time.sleep(0.1)
+        result = sa.handle("list")
+        assert "agent_a" in result
+        assert "agent_b" in result
+
+    def test_status(self, sa):
+        sa.handle("spawn x 某任务")
+        import time; time.sleep(0.1)
+        # 获取 agent ID
+        agent_id = list(sa.agents.keys())[0]
+        result = sa.handle(f"status {agent_id}")
+        assert "名称" in result
+
+    def test_kill(self, sa):
+        sa.handle("spawn y 要终止的任务")
+        import time; time.sleep(0.1)
+        agent_id = list(sa.agents.keys())[0]
+        result = sa.handle(f"kill {agent_id}")
+        assert "已终止" in result
+
+    def test_clean(self, sa):
+        sa.handle("spawn z 清理测试")
+        import time; time.sleep(0.1)
+        result = sa.handle("clean")
+        assert "已清理" in result
+
+    def test_invalid_agent(self, sa):
+        result = sa.handle("status nonexistent")
+        assert "未找到" in result
+
+
 # ══════════════════════════════════════
 #  引擎加载测试
 # ══════════════════════════════════════