Add Agents.md onboarding guide

[aviaviavi]

Summary

• add an agents playbook covering layout, workflow, and release steps
• document runtime toggles, testing coverage, and ready-to-ship checklist

Testing

• not run (docs only)

[aviaviavi]

✅ Reviewed the Agents.md addition for issue #3. The doc covers the repo layout, workflows, runtime toggles, testing guidance, and release steps, so the issue requirement to introduce an agents guide is satisfied. No edge cases or error messaging concerns since this is guidance-only documentation. No further changes requested.