fix: allow harness codex command override

scripts/execute.py

@@ -405,6 +405,9 @@ class StepExecutor:
 
     @staticmethod
     def _codex_command() -> str:
+        override = os.environ.get("HARNESS_CODEX_COMMAND", "").strip()
+        if override:
+            return override
         return (
             shutil.which("codex.cmd")
             or shutil.which("codex.exe")

scripts/test_execute.py

@@ -296,6 +296,7 @@ class ExecuteRunnerSafetyTests(unittest.TestCase):
 
             with patch.object(execute.subprocess, "run", side_effect=fake_run) as run_mock:
                 with patch.object(executor, "_codex_command", return_value="codex.cmd"):
+                    with patch.dict(os.environ, {"HARNESS_CODEX_MODEL": ""}):
                         executor._invoke_codex(step, long_preamble)
 
             cmd = run_mock.call_args.args[0]
@@ -322,6 +323,12 @@ class ExecuteRunnerSafetyTests(unittest.TestCase):
         with patch.object(execute.shutil, "which", side_effect=fake_which):
             self.assertEqual(execute.StepExecutor._codex_command(), "C:/tools/codex.cmd")
 
+    def test_codex_command_uses_env_override_first(self):
+        execute = load_execute()
+
+        with patch.dict(os.environ, {"HARNESS_CODEX_COMMAND": "C:/app/codex.exe"}):
+            self.assertEqual(execute.StepExecutor._codex_command(), "C:/app/codex.exe")
+
     def test_codex_exec_command_uses_model_env_override(self):
         execute = load_execute()
         with tempfile.TemporaryDirectory() as tmp: