ci(pages): add Jampack optimization step

- Setup Node.js 22 with npm cache
- Cache .jampack directory using actions/cache@v4
- Run npm run optimize to process _site with Jampack

Rationale: shrink asset size in CI to improve load performance before Pages upload.

Author: yunseo-kim

.github/workflows/pages-deploy.yml

@@ -94,6 +94,23 @@ jobs:
           else
             echo "No changes to commit."
           fi
+      
+      - name: Setup Node.js
+        uses: actions/setup-node@v5
+        with:
+          node-version: '22'
+          cache: 'npm'
+      
+      - name: Cache Jampack
+        uses: actions/cache@v4
+        with:
+          path: '.jampack'
+          key: jampack-${{ github.run_id }}
+          restore-keys: |
+            jampack
+
+      - name: Optimize site
+        run: npm run optimize
 
       - name: Upload site artifact
         uses: actions/upload-pages-artifact@v4