Integrate MCP Agent CLI into main HF CLI
- Add 'hf agent' subcommand to main HF CLI (gracefully handles missing mcp extra)
- Populate inference._mcp.__init__.py to properly export Agent and MCPClient classes
- Update documentation to show both 'hf agent run' and 'tiny-agents run' commands
- Maintains backward compatibility with standalone 'tiny-agents' command
Co-authored-by: lucain <lucain@huggingface.co>
