Enforce per-execution timeout via shell timeout command

The sandbox lifecycle timeout (timeout_secs) is a safety net, not the
per-execution limit. Use `timeout N` in the shell command to kill the
python process after exactly N seconds, matching the old subprocess.run
timeout behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: madiator

src/bespokelabs/curator/code_executor/code_execution_backend/sandbox_backend.py

@@ -87,7 +87,7 @@ def _execute_in_sandbox(
             sb.write_file(f"{WORKSPACE_DIR}/program.py", code)
             sb.write_file(f"{WORKSPACE_DIR}/input.txt", code_input)
 
-            result = sb.execute_command("bash", args=["-c", f"cd {WORKSPACE_DIR} && python program.py < input.txt"])
+            result = sb.execute_command("bash", args=["-c", f"cd {WORKSPACE_DIR} && timeout {timeout} python program.py < input.txt"])
 
             files = _collect_sandbox_files(sb)