ci: add GitHub Actions workflow for typecheck, build, and tests

.github/workflows/ci.yml

@@ -0,0 +1,45 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  ci:
+    name: Typecheck, build & test (Node ${{ matrix.node }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        node: [20, 22]
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Node ${{ matrix.node }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node }}
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Typecheck
+        run: npx tsc -b
+
+      - name: Build
+        run: npm run build
+
+      # No test script exists on main yet; #39 adds Vitest. --if-present keeps
+      # this step green today and picks the suite up automatically on merge.
+      - name: Test
+        run: npm run --if-present test