Add an official plugin guidelines to Omarchy native plugins including security and performance

[ronald2wing]

Use past Omarchy marketplace issue reviews to create a security and performance guideline that AI can parse in a website and the official plugins.
Since AI will parse the native Omarchy plugins, it will read their AGENTS.md to understand the guidelines. Also, having an official guideline will make it easier for the AI to find and parse it before creating any plugins.

One problem I am having right now is a lot of security issues, like permission checks and search query size limits. Having them in a guideline will make people follow them before making an issue.