feat: add System Monitor plugin for hardware telemetry

[luckyson113]

Purpose

This PR introduces the SystemMonitorPlugin to the OM1 sensor suite. Monitoring hardware health (CPU Load, RAM, and Thermal) is essential for agent stability, especially when deployed on edge devices like Raspberry Pi or Jetson.

Changes

• New Plugin: Created src/plugins/sensors/system_monitor.py.
• Telemetry Data: Provides real-time tracking of CPU usage, Memory availability, and Core temperature.
• Cross-platform Support: Implemented mock_mode and robust exception handling for environments where hardware sensors are unavailable (Windows/MacOS/Docker).
• Code Quality: Used standard logging for traceability and Complies with Ruff and Black formatting standards (fixed import sorting and string quotes) with English documentation.

Checklist

• I have forked the repository and created a new branch (feat/system-monitor-plugin).
• My code follows the project's coding style.
• I have included detailed English comments for international maintainers.
• The plugin handles missing hardware sensors gracefully.

Screenshots/Logs (Optional)

[System Monitor] CPU: 12.5% | RAM: 45.2% | Status: Healthy