Update AGENTS.md

[razinbouzar]

AGENTS.md is now a shorter operational guide instead of a broad contributor doc. It keeps the important repo map, replaces the old “all files need headers” rule with guidance based on make rat, centralizes generated-file ownership under a new “Generated Artifacts” section, and clarifies that make docker-build is a full verification-plus-build target rather than a cheap smoke test.

I also tightened the PR guidance so it matches .github/pull_request_template.md, including the requirement to add comments explaining the “why” when intent is not obvious, and trimmed the pitfalls list down to the mistakes most likely to confuse an agent.

Fixes #XXXX.

Description

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This PR has:

• been tested on a real K8S cluster to ensure creation of a brand new Druid cluster works.
• been tested for backward compatibility on a real K*S cluster by applying the changes introduced here on an existing Druid cluster. If there are any backward incompatible changes then they have been noted in the PR description.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

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Key changed/added files in this PR

• MyFoo
• OurBar
• TheirBaz

[razinbouzar]

@AdheipSingh or @abhishekrb19 can either of you review this? It's a concatenated version of the file in keeping with best practices.