Add AGENTS.md

[chrishavlin]

Here's a draft of an AGENTS.md (#5543 ). Branch is based off of #5541 , I'll keep this in draft until that is merged.

In terms of what I put in: I iterated with claude a bit with the following principles in mind:

• check CONTRIBUTING.md for developer guidelins
• no AI-attribution. AI is a tool, not an author.
• concise commenting (we don't need changelog-style content or AI-reasoning in comments, code should be self describing so only need comments when an experience developer would be confused)
• tool-specific config or repo mapping do not belong in AGENTS.md : we should keep AGENTS.md as a fairly static, simple file that directs AI style and general behaviors, not use it to map implementation details.

AI Disclosure

By submitting this pull request, I agree to adhere to yt's AI policy.

I used AI for (leaving both boxes unchecked certifies that no AI was used in these changes):

• low level assistance: code-completion, syntax lookup, explaining existing code.
• substantive generation: substantive portions of code and/or documentation were written by AI then edited or submitted as is. This includes copying code from an AI chat or agentic coding. I have read and can explain every changed line.

I used the following AI tools: Claude Fable (5.1)

The merge-base changed after approval.