Fix leftover yarn reference in README to npm

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ryu1kn

README.md

@@ -145,5 +145,5 @@ You can quickly mark the selected text by adding the `partial-diff` commands to
 1. Before you make a pull request, you can run linter and tests to avoid build failure
 
     ```sh
-    $ yarn run prep
+    $ npm run prep
     ```