feat: add advanced features - plugin system, security sandbox, metric…

…s, enhanced web UI

- Plugin architecture with external and built-in plugins (network, firewall)
- Advanced security: command sandboxing with resource limits and monitoring
- Usage metrics and analytics with persistent storage
- Enhanced LuCI web UI: real-time status, history, metrics dashboard
- Interactive REPL mode with command history and runtime configuration
- Setup wizard for guided initial configuration
- Comprehensive documentation updates

docs: add CHANGELOG and CONTRIBUTING; fix: make release script execut…

…able