Add Module system

[soymadip]

This PR introduces a module system for myctl.

Modules are loadable, trusted Bash components that extend myctl functionality while keeping the core code clean and maintainable.

Checklist

• Clear plugin entry function (myctl::<plugin>())

• Strict name spacing (no global functions/vars)

• Plugins separated from core code

• System + user plugin directories supported

• Deterministic plugin load order

• Cold-load rule (no logic at source time)

• Plugin metadata (name, description, permissions)

• Soft permission model (declared capabilities)

• Permission check before execution

• Guarded execution helpers (myctl_api::exec)

• Dangerous commands blocked or warned

• Core variables marked readonly

• Plugins listed in myctl help

• Clear security notice (plugins are trusted code)

Module Scoping

By default, modules have limited filesystem access
Module code:
/usr/lib/myctl/modules/screenshot/
~/.local/lib/myctl/modules/screenshot/

Module config:
~/.config/myctl/myctl.yml -> screenshot.*

Module cache / runtime data:
~/.cache/myctl/modules/screenshot/