OpenEnv 0.2.3 conformance: mount /openenv sub-app, add adapter + tests + example client

Level 2 open-ended env: auto-fuzzer + TaskCatalog + Hub-driven catalog + extended /reset

Apply paper-driven improvements: stratified verifier, sandbox hardening, coverage bonus, anti-hacking penalties, curriculum metadata

Overhaul env: per-episode state, ast.literal_eval probe parsing, sandboxed verifier with timeouts, 9 black-box functions, slim Dockerfile

initial commit