test: add comprehensive testing infrastructure

- Add npm test scripts for build, markdown, HTML, and link validation
- Install markdownlint-cli for content quality checks
- Install html-validate for HTML5 conformity validation
- Add htmltest (via devbox) for internal/external link checking
- Configure test rules in .htmltest.yml, .htmlvalidate.json, .markdownlint.json
- Add GitHub Actions workflow for automated CI testing
- Add TESTING.md documentation for test usage

.github/workflows/test.yml

@@ -0,0 +1,51 @@
+name: Hugo Site Tests
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
+        fetch-depth: 0
+
+    - name: Setup Hugo
+      uses: peaceiris/actions-hugo@v3
+      with:
+        hugo-version: 'latest'
+        extended: true
+
+    - name: Setup Node
+      uses: actions/setup-node@v4
+      with:
+        node-version: '24'
+        cache: 'npm'
+
+    - name: Install dependencies
+      run: |
+        npm ci
+        go install github.com/wjdp/htmltest@latest
+
+    - name: Run tests
+      run: |
+        npm run test:build
+        npm run test:markdown
+        npm run test:html
+        
+    - name: Run link checker
+      run: htmltest
+      continue-on-error: true
+
+    - name: Upload htmltest results
+      uses: actions/upload-artifact@v4
+      if: always()
+      with:
+        name: htmltest-report
+        path: tmp/.htmltest/