New AGENTS.md

[Gusarich]

We need to compose a new AGENTS.md to ensure consistent code guidelines with focus on maintainability and testability. Since we're using AI to write 100% of the code here, we need to make everything possible to decrease the rate of "errors" and simplify the job for AI when finding relevant parts and reading code and all that.

I see it as a strict guideline that not only ensures code is "good" but also ensures that code is easy for AI to navigate. The last part is important.