fix: harden subprocess output decoding across lessons

s15_integrated_harness/code.py

@@ -454,7 +454,7 @@ def _run_git(args: list[str], cwd: Path | None = None) -> tuple[bool, str]:
     try:
         result = subprocess.run(
             ["git", *args], cwd=cwd or WORKDIR,
-            capture_output=True, text=True, timeout=30,
+            capture_output=True, text=True, errors="replace", timeout=30,
         )
     except (OSError, subprocess.TimeoutExpired) as exc:
         return False, f"{type(exc).__name__}: {exc}"
@@ -903,7 +903,7 @@ def _run_bash_process(command: str, cwd: Path | None = None) -> tuple[str, int |
         process = subprocess.Popen(
             command, shell=True, cwd=cwd or WORKDIR,
             stdout=subprocess.PIPE, stderr=subprocess.PIPE,
-            text=True, start_new_session=True,
+            text=True, errors="replace", start_new_session=True,
         )
         with _shell_process_lock:
             _shell_processes.add(process)