chore: migrate Prettier workflow to use pnpm and update lockfile

Author: ToshiakiMasumori

.github/workflows/prettier.yml

@@ -20,17 +20,22 @@ jobs:
           # Important, so the commit can be pushed back
           token: ${{ secrets.GITHUB_TOKEN }}
 
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
           node-version: 20
-          cache: 'npm'
+          cache: 'pnpm'
 
       - name: Install dependencies
-        run: npm ci
+        run: pnpm install --frozen-lockfile
 
       - name: Run Prettier
-        run: npx prettier --write .
+        run: pnpm exec prettier --write .
 
       - name: Commit changes
         uses: stefanzweifel/git-auto-commit-action@v5