Add markdown linting tools and scripts

- Create package.json to manage markdownlint-cli2 as a dev dependency.
- Add PowerShell script for linting Markdown files with options for fixing issues.
- Update shell script to prefer locally installed markdownlint-cli2 or fallback to npx.

.github/workflows/ci.yml

@@ -14,6 +14,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 24
+          cache: npm
+
+      - name: Install pinned linter
+        run: npm ci
+
       - name: Run markdownlint
         run: bash scripts/lint-markdown.sh