A Rector extension for Symfony Mate

The new Rector extension for Symfony Mate splits one tool into two, and the split is the whole point. rector-preview runs with --dry-run enforced and cannot write, and its description says so in the one place the agent actually reads. Changing files means reaching for rector-apply, a separate tool that announces itself as a write operation, so inspecting code is never the thing that changes it. Rector still decides the refactoring, the same way every time, which is the part I am comfortable letting an agent reach for. It was contributed by mdjdev rather than by me, and that is the part of MatesOfMate that was always the point.