fix: add medea-eval entrypoint module

[SufianTA]

Summary

• add the missing medea.cli module referenced by the medea-eval console script
• delegate to the existing main.py evaluation CLI used in the README
• return a clear error if the source checkout entrypoint is unavailable

Validation

• python -m compileall medea main.py utils.py
• python -c "import importlib.util; spec=importlib.util.spec_from_file_location('medea_cli','medea/cli.py'); mod=importlib.util.module_from_spec(spec); spec.loader.exec_module(mod); assert callable(mod.main); print('cli wrapper ok')"
• git diff --check

Note: Direct from medea.cli import main requires the full Medea runtime dependencies because medea/init.py imports core modules at package import time.