ci: add GitHub Actions workflow (#26)

* ci: add GitHub Actions workflow for PRs and pushes to main

Runs typecheck, lint, tests, and build on Node 20 and 22.
format:check is deliberately omitted until existing prettier
failures are fixed.

* ci: add format:check step

The prettier failures are fixed in #25, which should merge first.

* ci: test Node 18 to cover minimum supported engine

package.json declares engines.node >=18, and all runtime and dev
dependencies support 18, so CI should exercise it.

Author: ben-vargas

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        node-version: [18, 20, 22]
+    name: Node ${{ matrix.node-version }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: npm
+
+      - run: npm ci
+
+      - run: npm run format:check
+
+      # typecheck + lint + test
+      - run: npm run ci
+
+      - run: npm run build