style(521): lint/type cleanup on decisions_lib parser + test

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/decisions_lib.py

@@ -13,6 +13,7 @@ block as its first non-blank content:
 
 An H2 with no metadata line is treated as status `legacy-unmigrated` (migration target).
 """
+
 from __future__ import annotations
 
 import re

scripts/tests/test_decisions_lib.py

@@ -1,4 +1,5 @@
 from pathlib import Path
+
 import scripts.decisions_lib as dl
 
 FIX = Path(__file__).parent / "fixtures" / "sample_decisions.md"
@@ -14,7 +15,9 @@ def test_parses_migrated_record():
     assert r.supersedes == "none"
     assert r.superseded_by == "none"
     assert r.rule == "Every CI services container gets an explicit cap."
+    assert r.signals is not None
     assert "issues: #390 #406" in r.signals
+    assert r.mechanics is not None
     assert r.mechanics.startswith("docs/ci-cd.md")