test: close unclosed file handles in tests (Path read/write)

Replace bare open().read()/write() in tests with Path.read_text/
write_text (or json.loads over read_text); the five remaining
open(...).close() empty-file creations become Path.write_text('').
No assertion, predicate, or production code changed; no behaviour
change beyond closing the handle deterministically.

Long-function splits and server.py module split intentionally not
included: the longest functions (_dispatch, _create_task,
_dispatch_project, ~375-395 lines in a 10430-line server.py) are
central routing/creation paths, and splitting them is not low-risk;
see PR body.

tests/test_host_thread_cli.py

@@ -252,7 +252,7 @@ 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.load(open(state_path, encoding="utf-8"))
+            saved = json.loads(Path(state_path).read_text(encoding="utf-8"))
             self.assertEqual(saved["tasks"]["t1"]["status"], "archived")
             calls = self._calls_before_close(fake)
             self.assertEqual(calls, [("archive", "th-1")])