docs: the ISO repo's PRs and vmisobuild.sh target master (feat/steamify is gone)

.claude/skills/steamify-branch-cleanup/SKILL.md

@@ -12,7 +12,7 @@ Rules:
 - Never touch unmerged branches; list them and report instead.
 - Never commit or merge to main.
 
-Merging a feature/bugfix PR yourself (into a release branch, or `feat/steamify` in the ISO repo): delete the
+Merging a feature/bugfix PR yourself (into a release branch, or `master` in the ISO repo): delete the
 branch at once, remote and local (`gh pr merge N --merge --delete-branch`, then `git branch -D`, `git fetch
 --prune`); the user asked for this on 2026-09-29. Release branches are never deleted (kept as history next to
 their tags). Unmerged old branches (`feat/vram-booster`, `refactor/qml-screens`, `backup/*`) are reported, not