fix(ci): update API documentation validation for Markdown format

- Replace HTML validation with Markdown file checks
- Validate index.md, modules.md, and classes directory
- Fixes CI failure after TypeDoc → Markdown migration (Story 6.3)

Related to: Story 6.9 (GitHub Actions Deployment)

Author: salacoste

.github/workflows/ci.yml

@@ -71,9 +71,10 @@ jobs:
         if: matrix.node-version == '20.x'
         run: |
           test -d docs/api || exit 1
-          test -f docs/api/index.html || exit 1
-          test -f docs/api/assets/search.js || exit 1
-          echo "API documentation validated successfully"
+          test -f docs/api/index.md || exit 1
+          test -f docs/api/modules.md || exit 1
+          test -d docs/api/classes || exit 1
+          echo "API documentation (Markdown) validated successfully"
 
       - name: Upload API docs artifact
         if: matrix.node-version == '20.x'