feat(graders): implement easy_grader — pure attack success rate scoring

graders/easy_grader.py

@@ -3,5 +3,9 @@ Easy Grader — Pure Success Rate
 Evaluates only if the attacker reached the breakthrough threshold.
 """
 def grade_easy(history: list[dict]) -> float:
-    """Scaffold for easy grader interface."""
-    return 0.0
+    if not history:
+        return 0.0
+    
+    success_rates = [h.get("attack_success_estimate", 0.0) for h in history]
+    max_success = max(success_rates)
+    return max_success