chore(deps): use local markdownlint in CI

Move markdownlint-cli into tracked npm devDependencies so CI installs pinned project dependencies with `npm ci` and runs the local binary via `npx --no-install`. Also ignore `node_modules` now that the repo manages Node-based tooling.

Author: ReenigneArcher

.github/workflows/ci.yml

@@ -18,12 +18,12 @@ jobs:
       - name: Checkout
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
 
-      - name: Install markdownlint-cli
-        run: npm install --ignore-scripts -g markdownlint-cli
+      - name: Install dependencies
+        run: npm ci --ignore-scripts
 
       - name: Lint markdown files
         run: |
-          markdownlint \
+          npx --no-install markdownlint \
             -f \
             -o markdownlint_report.log \
             --ignore-path .markdownlintignore \

.gitignore

@@ -1,2 +1,5 @@
 # ignore JetBrains IDE files
 .idea/
+
+# ignore Node.js dependencies
+node_modules/