fix: update CI workflow to use npm instead of pnpm

The PR 444 migration to npm requires updating the workflow to use npm commands
and npm caching instead of pnpm. Removes the redundant pnpm setup step since
npm is built-in to the Node.js setup action.

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

Author: bdfinst

.github/workflows/ci.yml

@@ -20,17 +20,14 @@ jobs:
         with:
           submodules: recursive
 
-      - name: Setup pnpm
-        uses: pnpm/action-setup@v6
-
       - name: Setup Node.js
         uses: actions/setup-node@v6
         with:
           node-version: 24
-          cache: 'pnpm'
+          cache: 'npm'
 
       - name: Install dependencies
-        run: pnpm install --frozen-lockfile
+        run: npm ci
 
       - name: Setup Hugo
         uses: peaceiris/actions-hugo@v3
@@ -39,12 +36,12 @@ jobs:
           extended: true
 
       - name: Run linting tests
-        run: pnpm test
+        run: npm test
 
       - name: Build site with Hugo
         run: hugo -d $GITHUB_WORKSPACE/public
 
       - name: Run broken links test
-        run: pnpm run test:links
+        run: npm run test:links
         env:
           CI: true