ci: add PR workflow, make release fail loudly when registry is unreachable, document setup

.github/workflows/ci.yml

@@ -0,0 +1,36 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches-ignore:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build
+        run: npm run build
+
+      - name: Test
+        run: npm test
+
+      - name: Require a changeset
+        run: |
+          if [ -z "$(ls -A .changeset/*.md 2>/dev/null | grep -v README)" ]; then
+            echo "No changeset found. Run 'npx changeset' and commit the result so this change gets a version bump."
+            exit 1
+          fi