Add skills

skills/maintain-mutations/SKILL.md

@@ -0,0 +1,18 @@
+---
+name: Maintain mutations
+description: Trigger when asked to maintain mutations
+---
+
+Start with:
+
+```sh
+mutannot <path/to/test-project.fsproj> --validate
+```
+
+Fix every reported error. Re-run `mutannot <path/to/test-project.fsproj> --validate` until it succeeds with no errors.
+
+If you change mutations, you must run the changed mutations with:
+
+```sh
+mutannot <path/to/test-project.fsproj> --run <mutation id 1> <mutation id 2> <...>
+```