ci: add GitHub Actions workflow for lint, build, and audit

Triggers on push/PR to main. Runs npm ci, build:all:prod, and npm audit
on Node 20. No test step yet (test script is placeholder).

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

Author: vveerrgg

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: CI
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: 'npm'
+      - run: npm ci
+      - run: npm run build:all:prod
+      - run: npm audit