update-existing-branch

Refresh an existing .NET release-notes milestone branch or PR incrementally. Checks whether the VMR ref moved, regenerates `changes.json` only when needed, merges the delta into `features.json`, integrates new material into existing markdown clusters, and responds to review feedback. USE FOR: reruns on a populated release-notes branch. DO NOT USE FOR: first-pass generation of a new milestone (use generate-changes, generate-features, and release-notes).