AGENTS.md Architecture Map is missing notebook-migration-service

[Yicong-Huang]

Task Summary

notebook-migration-service/ is a JVM module wired in build.sbt but missing from the Backend services row of the Architecture Map in AGENTS.md. This addition was originally part of #6113 but was dropped there so the release/v1.2 backport applies cleanly (the service does not exist on that branch). Re-add it on main only.

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other