feat(graders): add medium_grader scaffold with interface

graders/medium_grader.py

@@ -0,0 +1,7 @@
+"""
+Medium Grader — Success + Diversity
+Evaluates breakthrough and variety of strategies used.
+"""
+def grade_medium(history: list[dict]) -> float:
+    """Scaffold for medium grader."""
+    return 0.0