Fix GitHub Pages deployment: switch from pnpm to npm

The workflow was using pnpm but we installed packages with npm,
causing lockfile mismatch errors in CI. Switched to npm for consistency.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: testac974

.github/workflows/deploy-pages.yml

@@ -28,25 +28,20 @@ jobs:
         with:
           fetch-depth: 0
 
-      - 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: 'pnpm'
-          cache-dependency-path: 'website/pnpm-lock.yaml'
+          cache: 'npm'
+          cache-dependency-path: 'website/package-lock.json'
 
       - name: Install dependencies
         working-directory: ./website
-        run: pnpm install --frozen-lockfile
+        run: npm ci
 
       - name: Build Next.js site
         working-directory: ./website
-        run: pnpm build
+        run: npm run build
         env:
           NEXT_PUBLIC_BASE_PATH: /agentic-coding-book