Add initial AGENTS.md / CLAUDE.md file #20941

Is your feature request related to a problem or challenge?

Now that AI models seem to (partly) author many PRs, let's add some infra for that as well.

Describe the solution you'd like

Just start with some markdown files, point to the contributor / architecture docs and add some hints on doing the formatting / clippy / testing to help saving time.

Describe alternatives you've considered

No response

Additional context

No response