extend AGENTS.md based on CONTIBUTING.md

[mshustov]

Summary

CONTIBUTING.md contains a lot of practical instructions for agents as well

[github-actions]

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[Copilot]

Pull request overview

This PR updates the contributor/agent guidance docs so that AGENTS.md includes more of the practical “how to work here” guidance currently found in CONTRIBUTING.md, and removes a stale checklist link from CONTRIBUTING.md.

Changes:

• Expand AGENTS.md with additional policies, scope/testing expectations, and closeout guidance derived from CONTRIBUTING.md.
• Remove a reference to a high-level changes checklist from CONTRIBUTING.md.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
CONTRIBUTING.md	Removes a stale checklist reference; minor doc wording remains.
AGENTS.md	Adds more explicit agent workflow/policy guidance (scope discipline, testing expectations, closeout checklist).