refactor: full openenv protocol compliance

refactor: move all root-level files to repo root

fix: make POST /reset body optional for OpenEnv validator compatibility

fix: packaging, grading baseline, action registry, observations, tests, deploy config

feat: Phase 1 - cross-column validation, observation enrichment, undo, profile

fix: pass openenv validate - add uv.lock, server entry point, openenv-core dep

fix: smart fill_missing strategy - mode for categorical, median for numeric

feat: Unit 13 - expert task integration with ML metadata

feat: Units 11+12 - configurable difficulty, procedural gen, ML metadata

feat: Units 9+10 - cost-aware rewards, richer noise taxonomy

feat: Units 7+8 - sample rows, action history, 7 new action types

feat: Phase 1 + Unit 6 - deterministic grader, inference.py, Dockerfile, compliance fixes

feat: Data Cleaning RL Environment for OpenEnv Hackathon