fix: encode harness prompts as utf-8

tests/test_execute.py

@@ -0,0 +1,36 @@
+import json
+import subprocess
+
+from scripts import execute
+
+
+def test_invoke_codex_uses_utf8_for_unicode_prompt(tmp_path, monkeypatch):
+    phase_dir = tmp_path / "phases" / "unicode-phase"
+    phase_dir.mkdir(parents=True)
+    (phase_dir / "index.json").write_text(
+        json.dumps(
+            {
+                "project": "FESA Structural Solver",
+                "phase": "unicode-phase",
+                "steps": [{"step": 0, "name": "unicode", "status": "pending"}],
+            }
+        ),
+        encoding="utf-8",
+    )
+    (phase_dir / "step0.md").write_text("Euler–Bernoulli 보", encoding="utf-8")
+
+    invocation = {}
+
+    def capture_run(command, **kwargs):
+        invocation["command"] = command
+        invocation["kwargs"] = kwargs
+        return subprocess.CompletedProcess(command, 0, stdout="", stderr="")
+
+    monkeypatch.setattr(execute, "ROOT", tmp_path)
+    monkeypatch.setattr(execute.subprocess, "run", capture_run)
+
+    executor = execute.StepExecutor("unicode-phase")
+    executor._invoke_codex({"step": 0, "name": "unicode"}, "승인된 설계 – ")
+
+    assert "Euler–Bernoulli 보" in invocation["kwargs"]["input"]
+    assert invocation["kwargs"]["encoding"] == "utf-8"