docs: add AGENTS.md

[kpal81xd]

Summary

• Adds AGENTS.md documenting conventions, architecture, and best practices for AI agents and developers working on the editor frontend.
• Covers project overview, page loading lifecycle, directory structure, the Caller pattern, editor-api module, build system, testing, code style, common patterns, and anti-patterns.

Details

This is a documentation-only change — no code modifications. The file serves as a reference for both human contributors and AI coding agents to understand how the editor is structured and how to make changes correctly.

[willeastcott]

My gut feeling is that this is needlessly wordy. AFAIK, your AGENTS.md is going to used up some of your context window. If we have a workspace referencing all or most of the PlayCanvas repos, all the AGENTS.md files are going to add up quickly. My approach is to get AI to write the skinniest AGENTS.md possible and then add to it whenever the LLM gets something wrong. See the developer-site one for an example:

https://github.com/playcanvas/developer-site/blob/main/AGENTS.md

[kpal81xd]

My gut feeling is that this is needlessly wordy. AFAIK, your AGENTS.md is going to used up some of your context window. If we have a workspace referencing all or most of the PlayCanvas repos, all the AGENTS.md files are going to add up quickly. My approach is to get AI to write the skinniest AGENTS.md possible and then add to it whenever the LLM gets something wrong. See the developer-site one for an example:

https://github.com/playcanvas/developer-site/blob/main/AGENTS.md

Yep it is very long so im going to try condense it down