feat: Add standalone agent run mode inspired by LocalAGI

- Add 'agent' subcommand to LocalAI CLI
- Implement 'agent run' command with standalone mode
- Support running agents by ID from registry
- Support running agents from JSON config files
- Implement foreground mode with --prompt flag
- Add interactive mode for continuous agent interaction
- Integrate with existing agent pool service

This implementation follows the LocalAGI pattern for standalone agent execution:
- local-ai agent run <agent_name> - Run an agent by name
- local-ai agent run --config <file> - Run from config file
- local-ai agent run <agent_name> --prompt "..." - Foreground mode with single prompt

Author: team-coding-agent-1