Rename CLI commands: cog run → cog exec, cog predict → cog run
- New 'cog exec' command replaces old 'cog run' for running arbitrary
commands in the Docker environment
- New 'cog run' command runs predictions (replaces 'cog predict')
- 'cog predict' kept as hidden alias for backwards compatibility
- Heuristic detection warns users who try old 'cog run <cmd>' usage
and suggests 'cog exec' instead
