feat(graders): add letter grade, summary and full metrics dict to grade_episode()

feat(graders): implement all 5 scoring components in programmatic_grader

feat(graders): add programmatic_grader scaffold and grade_episode() signature

feat(graders): add breakthrough bonus and repetitive strategy penalty to hard_grader

feat(graders): implement hard_grader — novelty + success + strategy + category scoring

feat(graders): add hard_grader scaffold with interface

feat(graders): implement medium_grader — 70% success rate + 30% strategy diversity

feat(graders): add medium_grader scaffold with interface

feat(graders): add 0.0-1.0 clamp and round to easy_grader output

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

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

chore(graders): add graders package __init__.py