ci: run lint:format on branches

.github/workflows/test-markdown.yml

@@ -0,0 +1,23 @@
+name: Lint Markdown
+
+# run CI on pushes to main, and on all PRs (even the ones that target other
+# branches)
+
+on:
+  push:
+    branches: [$default-branch]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [18.x]
+    steps:
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v3
+        with:
+          node-version: ${{ matrix.node-version }}
+      - name: Lint
+        run: yarn lint:format