add AGENTS.md to test-infra repo

[kannon92]

Adding agents.md to test-infra repo.

This gives context to AI assistants and helps guide how to develop in this repo.

I generated this with claude init and moved that generated file to AGENTS.md.

Gemini and Claude are able to read the canoical AGENTS.md file with the @AGENTS.md.

I've added this into kueue and jobset.

[BenTheElder]

I worry about having yet another set of disjoint docs to maintain, can we just tell it to read the README and the Makefile and add any missing details to those?

I think there's way too much info in here that will fall out of date as people that don't use these tools iterate on the makefiles etc.

Can we keep this to info that doesn't make sense for README.md, Makefile help text, developers docs, etc, IE acutal agent-specific instructions? (Such as pointing it to read in those files?)

[SergeyKanzhelev]

lgtm with @BenTheElder 's comments.

I found it best to start with some agents.md and then iterate on it. I wonder if we should place AGENTS_IMPLEMENTATION.md in some subfolders with AGENTS.md linnking it, so we can add more owners for it.

[SergeyKanzhelev]

I wonder how much we will need to keep this section in root vs. inside the config/jobs/

[kannon92]

Yea I think a sub agents folder for config/jobs (espeically sig-node) could be helpful.

[SergeyKanzhelev]

/lgtm

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[kannon92]

#35820 (review)

@BenTheElder I updated this to tell agents file to read README.md and contributor guides as much as possible.

PTAL.