feat(test): add Vitest test suite and GitHub Actions workflow

28 tests covering LocalStore (CRUD, version conflicts, checklist nesting,
dependency cycles, E2E flow) and RemoteStore (success paths, 404/409/5xx
error mapping, timeout with status 504, network failure).

GitHub Actions runs tests on push to master and all PRs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chapterjason

.github/workflows/test.yml

@@ -0,0 +1,30 @@
+name: Test
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+concurrency:
+  group: test-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: Run tests
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version: 24
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run tests
+        run: npm test