feat(cli): add global plugin foundation

[codeaholicguy]

Summary

Adds the AI DevKit global npm plugin foundation.

Users can install npm-based plugins globally and run plugin-contributed CLI commands:

• ai-devkit plugin add <package>
• ai-devkit plugin remove <package>
• ai-devkit plugin list
• ai-devkit <plugin-command>

Plugins install into ~/.ai-devkit/npm/node_modules and are enabled through ~/.ai-devkit/.ai-devkit.json.

What Changed

• Added plugin add/remove/list CLI commands.
• Added global plugins config support.
• Added npm install/uninstall service for global plugins.
• Added package.json aiDevkit.commands manifest validation.
• Added plugin command loading and Commander registration at CLI startup.
• Added initial plugin runtime API:
  • cwd
  • homeDir
  • configPath
  • getConfig()
  • getMemoryDbPath()
  • logger