docs: remove deleted project docs skill requirement

tools/repository-hygiene.test.mjs

@@ -117,7 +117,7 @@ test('root contains only governed long-lived entries', async () => {
   }
 });
 
-test('maintain-project-docs is the sole active project memory', async () => {
+test('project docs are the sole active project memory', async () => {
   const projectDocs = path.join(ROOT, '.project-docs');
   const required = [
     '00-brief/project-positioning.md',