Convert to Claude Code plugin for automatic version check hook

[simons-plugins]

Currently the skill relies on a skill.md instruction telling Claude to WebFetch the VERSION file on each /control-page invocation. This works but is slower and less reliable than a proper SessionStart hook.

The GSD plugin (superpowers) distributes a background version check via its plugin hooks/ directory and hooks.json, so all users get it automatically on session start.

Proposed change:

• Restructure as a Claude Code plugin with hooks.json + session-start hook
• Background check compares local VERSION against raw.githubusercontent.com/.../main/VERSION
• Users install via claude plugins add instead of manual git clone
• Existing skill.md WebFetch fallback can remain for non-plugin installs