fix: close bare open() in host.py and ResourceWarning in test_host_thread_cli

- host.py: 7 one-shot reads switched to with open (run_health_check
  pidfile, daemon-status pid/log, daemon-stop pid, daemon old pid,
  interrupt poll state, parallel jobs); read content, encoding args
  and error semantics unchanged.
- tests/test_host_thread_cli.py: test_old_archive_unchanged state
  read switched to with open; added a warnings-capture assertion
  scoped to the state read so a regression fails the test.
- Intentionally untouched: daemonize log handle (daemon-lifetime
  dup2 target) and the two _load_config_file / init sites owned
  by PR #266.

tests/test_host_thread_cli.py

@@ -1,9 +1,11 @@
+import gc
 import io
 import json
 import os
 import sys
 import tempfile
 import unittest
+import warnings
 from contextlib import redirect_stdout
 from unittest import mock
 import host
@@ -246,7 +248,19 @@ class ThreadCliTest(unittest.TestCase):
                             code = e.code if isinstance(e.code, int) else 1
             self.assertEqual(code, 0)
             self.assertIn("archived t1", out.getvalue())
-            saved = json.loads(Path(state_path).read_text(encoding="utf-8"))
+            # 第 4 项回归：此处裸 open() 曾在 GC 时报 ResourceWarning；
+            # 只捕获本段读 state 的警告断言（_load_config_file 的
+            # 警告属 #266 轨，不在此断言范围内）。
+            with warnings.catch_warnings(record=True) as caught:
+                warnings.simplefilter("always", ResourceWarning)
+                with open(state_path, encoding="utf-8") as f:
+                    saved = json.load(f)
+                gc.collect()
+            self.assertEqual(
+                [w for w in caught
+                 if issubclass(w.category, ResourceWarning)
+                 and "host-state.json" in str(w.message)],
+                [])
             self.assertEqual(saved["tasks"]["t1"]["status"], "archived")
             calls = self._calls_before_close(fake)
             self.assertEqual(calls, [("archive", "th-1")])