fix(host): _load_config_file 与 init 的 open() 改用 with 关闭

行号纠正：摘要的 2199/2443 是另一基线行号；origin/main
aebc623 上两处是 _load_config_file 的 json.load(open(path))
与 main() init 分支的 json.dump(init_cfg, open(...))，
2443 行在 aebc623 上是 if not args.repo: 并无 open。
只改资源关闭：open 参数、读写内容、错误语义（坏 JSON 仍
转 RuntimeError）均不变，不加 encoding、不改其它裸 open()。
回归：ConfigFileCloseTest（3 项），旧代码上 3 项全 fail、
新代码上全 pass（warnings 过滤 + gc 证明无 ResourceWarning）。

tests/test_host_bugfix_host.py

@@ -80,5 +80,78 @@ class ReadLoopNonDictTest(unittest.TestCase):
         self.assertEqual(srv.events[0]["method"], "thread/started")
 
 
+class ConfigFileCloseTest(unittest.TestCase):
+    """第 2 项：_load_config_file 与 init 的 open() 必须关闭。
+
+    行号纠正：任务摘要的 2199/2443 是另一基线（coverage85
+    worktree）的行号；origin/main aebc623 上两处分别是
+    _load_config_file 的 json.load(open(path)) 与 main() init
+    分支的 json.dump(init_cfg, open(config_path, "w"))，
+    2443 行在 aebc623 上是 `if not args.repo:`，并无 open。
+    本测试只针对这两处生产路径，不扩大到其它裸 open()。
+    """
+
+    def test_load_config_file_no_resource_warning(self):
+        import gc
+        import os
+        import tempfile
+        import warnings
+        with tempfile.TemporaryDirectory() as tmp:
+            cfg_path = os.path.join(tmp, "cfg.json")
+            with open(cfg_path, "w", encoding="utf-8") as f:
+                json.dump({"model": "x"}, f)
+            with warnings.catch_warnings(record=True) as caught:
+                warnings.simplefilter("always", ResourceWarning)
+                value = host._load_config_file(cfg_path)
+                gc.collect()
+            self.assertEqual(value, {"model": "x"})
+            self.assertEqual(
+                [str(w.message) for w in caught
+                 if issubclass(w.category, ResourceWarning)], [])
+
+    def test_load_config_file_bad_json_still_raises_and_closes(self):
+        import gc
+        import os
+        import tempfile
+        import warnings
+        with tempfile.TemporaryDirectory() as tmp:
+            cfg_path = os.path.join(tmp, "cfg.json")
+            with open(cfg_path, "w", encoding="utf-8") as f:
+                f.write("{not json")
+            with warnings.catch_warnings(record=True) as caught:
+                warnings.simplefilter("always", ResourceWarning)
+                with self.assertRaises(RuntimeError):
+                    host._load_config_file(cfg_path)
+                gc.collect()
+            self.assertEqual(
+                [str(w.message) for w in caught
+                 if issubclass(w.category, ResourceWarning)], [])
+
+    def test_init_writes_config_no_resource_warning(self):
+        import gc
+        import os
+        import tempfile
+        import warnings
+        from unittest import mock
+        with tempfile.TemporaryDirectory() as tmp:
+            cfg_path = os.path.join(tmp, "new-config.json")
+            argv = ["host.py", "--config", cfg_path, "init"]
+            out = io.StringIO()
+            with warnings.catch_warnings(record=True) as caught:
+                warnings.simplefilter("always", ResourceWarning)
+                with mock.patch.object(sys, "argv", argv):
+                    with contextlib.redirect_stdout(out):
+                        host.main()
+                gc.collect()
+            with open(cfg_path, encoding="utf-8") as f:
+                written = json.load(f)
+            self.assertEqual(written["state"], "host-state.json")
+            self.assertEqual(written["model"], "gpt-5.6-terra")
+            self.assertIn("配置文件已写", out.getvalue())
+            self.assertEqual(
+                [str(w.message) for w in caught
+                 if issubclass(w.category, ResourceWarning)], [])
+
+
 if __name__ == "__main__":
     unittest.main()