ci: add GitHub Actions workflow (build + test on Node 18/20/22)

Runs TypeScript compilation and all 49 tests on every push/PR to main.
Tests across Node.js 18, 20, and 22 for compatibility verification.

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

Author: chrishooooo-netizen

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [18, 20, 22]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+
+      - name: Install dependencies
+        working-directory: packages/trail-core
+        run: npm install
+
+      - name: Build
+        working-directory: packages/trail-core
+        run: npx tsc
+
+      - name: Run tests
+        working-directory: packages/trail-core
+        run: node --test dist/test/roundtrip.test.js