chore: fix ruff lint warnings and signal handling edge cases

- Fix pre-existing ruff lint warnings (F401, F541, F841, E741)
- Exclude .md/.json/.yml from ruff format check
- Unblock SIGTERM/SIGINT via pthread_sigmask in early signal handler
- Do not restore SIG_DFL on unregister to prevent pending signal kills

scripts/eval/evaluate_mmlu.py

@@ -215,7 +215,6 @@ def _permute_choices(item: dict, rng: random.Random) -> tuple[dict, str]:
     positional bias (e.g. always picking B).
     """
     letters = ("A", "B", "C", "D")
-    contents = [item[k] for k in letters]
     perm = list(letters)
     rng.shuffle(perm)
     permuted = {"question": item["question"]}