Fix Phase 2 OpenEnv validation traps: add grader paths to openenv.yaml and safe parameterless defaults

Spec-compliance overhaul: remove difficulty_multiplier, weighted blend scoring, dep_hard fix, [END] format

Major grading overhaul: difficulty multiplier, tighter scoring, mastery removal, precision penalties

fix(benchmark): Hardening multi-agent environment and strict score compliance

Clamp scores strictly to (0.01, 0.99) to pass OpenEnv Phase 2 continuous environment score verification checks

first commit