Add .github/agents.md: guidance for AI-assisted PRs (closes #6564)

[Ave19Anurag]

Description (required)

Fixes #6564

What changes did you make and why?

Summary Adds .github/agents.md with guidance for AI/code-generation and human contributors.

Motivation Provide clear, consistent guidance so generated PRs are higher quality and easier for maintainers to review.

Implementation New .github/agents.md file; no code changes.

Tests performed (required)

Review the .github/agents.md content on GitHub. CI should run; since no code changes, CI should pass quickly.

I have read CONTRIBUTING.md.