feat(graders): add easy_grader scaffold with docstring and interface

graders/easy_grader.py

@@ -0,0 +1,7 @@
+"""
+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