feat: Add standalone agent run mode inspired by LocalAGI (#9056)

- Add 'agent' subcommand with 'run' and 'list' sub-commands
- Support running agents by name from pool.json registry
- Support running agents from JSON config files
- Implement foreground mode with --prompt flag for single-turn interactions
- Reuse AgentPoolService for consistent agent initialization
- Add comprehensive unit tests for config loading and overrides

Fixes #8960

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@noreply.github.com>