Add official System Monitor plugin

[charpeni]

This is a proposal to add System Monitor as an official plugin, please close this pull request if you believe it shouldn't be part of the official/built-in plugins and I will move this to my repository.

I've been running bb on a VM and I've been enjoying this plugin architecture, I figured it could be a good idea to show stats of the host machine, especially the remaining space.

Desktop	Mobile

Summary

• Add System Monitor as an install-on-demand official plugin under the Host access category
• Provide a responsive sidebar panel for CPU usage and speed, memory, disk space, load average, uptime, and host details
• Refresh metrics every five seconds and handle mobile sidebar-button spacing
• Add bb system-monitor [show] [--json] for human-readable and structured output
• Document the plugin across the CLI guide, agent skill, configuration, and official plugin release process

System Monitor reports metrics for the host running the bb server, not separately enrolled execution machines. CPU speed uses os.cpus() with a Linux /proc/cpuinfo fallback.