feat(ci): Add basic CI workflow with OS matrix

- Run lint, typecheck, and test on ubuntu, macos, and windows
- Use Node.js 20 with npm caching
- All steps have explicit timeouts to prevent hangs

Author: ProjectLiminality

.github/workflows/ci.yml

@@ -0,0 +1,45 @@
+name: CI
+
+on:
+  push:
+    branches: [main, 'epic/*', 'feature/*']
+  pull_request:
+    branches: [main]
+
+jobs:
+  check-all:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 15
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        timeout-minutes: 2
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: 'npm'
+        timeout-minutes: 3
+
+      - name: Install dependencies
+        run: npm ci
+        timeout-minutes: 5
+
+      - name: Run lint
+        run: npm run lint
+        timeout-minutes: 2
+
+      - name: Run typecheck
+        run: npm run typecheck
+        timeout-minutes: 3
+
+      - name: Run tests
+        run: npm run test
+        timeout-minutes: 5