fix(ci): use npm for markdownlint-cli installation

markdownlint-cli is a Node.js package, not Python. Also removed
|| true to ensure CI fails on actual lint errors.

Author: jeremyeder

.github/workflows/ci.yml

@@ -13,23 +13,22 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python 3.11
-        uses: actions/setup-python@v5
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
         with:
-          python-version: "3.11"
+          node-version: "20"
 
       - name: Install dependencies
         run: |
-          pip install markdownlint-cli
+          npm install -g markdownlint-cli @mermaid-js/mermaid-cli
 
       - name: Lint markdown files
         run: |
-          markdownlint docs/**/*.md README.md CLAUDE.md || true
+          markdownlint docs/**/*.md README.md CLAUDE.md
 
       - name: Validate Mermaid diagrams
         run: |
-          npm install -g @mermaid-js/mermaid-cli
-          ./scripts/validate-mermaid.sh || true
+          ./scripts/validate-mermaid.sh
 
       - name: Check documentation structure
         run: |