Add __all__ export to task_graders.py for proper validator discovery of graders

refactor: revert to root-level package structure with proper imports and hacky pyproject.toml setup

refactor: proper package structure - move he_demo package to dedicated folder

fix: clamp grader scores strictly between 0 and 1 for validator compliance

Fix Phase 2 validation: Add missing graders for advanced_efficiency and expert_optimization tasks

Fix: Make task graders explicitly discoverable with proper metadata and endpoints