Adding AGENTS.md for developers who wish to use AI to develop their games

[BrentFarris]

For those of you who use AI for coding, I've committed an AGENTS.md file with a distillation of how to generally use the engine. Those who do not use AI, you can also use this document to better understand the general usage and layout of the engine.

Though I am the AI developer for my job, I have hardly used AI to write anything in Kaiju, nearly all of it is hand written. So, I'm open to any suggestions, additions, or fixes to the AGENTS.md that will help improve AI's ability to work with the engine code.