Run typecheck, tests, and build on pull requests

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+
+# A new push to a PR makes the in-flight run obsolete; cancel it.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  verify:
+    name: Typecheck, test, build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 24
+          cache: yarn
+
+      - name: Install dependencies
+        run: yarn install --frozen-lockfile
+
+      - name: Typecheck
+        run: yarn typecheck
+
+      - name: Run tests
+        run: yarn test
+
+      # prepack runs this on publish, so a build break would otherwise
+      # only surface during the release run, after the merge has landed.
+      - name: Build
+        run: yarn build