fix: self-contained grader + remove FACTORY_TASK from Dockerfile

fix: lazy openenv.core imports so FactoryEnv works on all Python envs

fix: graders use FactoryEnv, pure-Python fallback if unavailable

fix: grader.py pure stdlib, no external deps, works on Python 3.11+

fix: zero-arg self-contained graders, inference runs all 3 tasks

fix: add spec_version:1 to openenv.yaml, add standalone root grader.py with score_easy/medium/hard