Add a root pnpm `merge-changelog <package> <stable-version>` maintenance command to make prerelease changelogs easy to rearrange. The command should edit `packages/<package>/CHANGELOG.md` in place using the repository's current changelog format. For the target stable release, combine its entries with the contiguous alpha, beta, and release-candidate sections for the same version, regroup all entries in the standard changelog category order, and remove the now-redundant standalone prerelease sections. Update the stable release's compare link so it starts at the previous stable release, retain links to each prerelease's tagged changelog file, and preserve the content before the target release and from the previous stable release onward.