docs(AGENTS.md): cross-reference remaining project README files

[jsonmp-k8]

Summary

• Follows up on docs(AGENTS.md): reference backend/README.md for backend work #13032 to add the remaining 7 README/CONTRIBUTING cross-references to AGENTS.md
• Each reference is placed in the section where it's most useful:
  • CONTRIBUTING.md (root) → Commit policy section
  • api/README.md → Regenerate protobufs section
  • backend/api/README.md → Regenerate protobufs section
  • frontend/README.md → Key paths section
  • kubernetes_platform/README.md → Key paths section
  • manifests/kustomize/README.md → Local cluster deployment section
  • test/README.md → Local testing section

Test plan

• Verify all referenced files exist
• Verify references render correctly in markdown

[google-oss-prow]

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[Copilot]

Pull request overview

Adds remaining internal documentation cross-references in AGENTS.md so contributors can quickly find the appropriate README/CONTRIBUTING guidance for common workflows (commit policy, protobuf regeneration, deployments, and testing).

Changes:

• Updates AGENTS.md “Last updated” date.
• Adds references to remaining repo READMEs/CONTRIBUTING docs in the most relevant sections (commit policy, deployment, testing, protobuf generation, and key paths).

[jsonmp-k8]

/ok-to-test