Use Node 20 only in CI workflow

- Remove Node 18 from test matrix
- Node 18 has issues with ESM TypeScript config files
- Node 20 is LTS and works correctly with Vitest ESM setup
- Fixes ERR_REQUIRE_ESM error in CI

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Jeffrey Lauwers

.github/workflows/ci.yml

@@ -15,21 +15,17 @@ jobs:
     name: Lint, Test & Build
     runs-on: ubuntu-latest
 
-    strategy:
-      matrix:
-        node-version: [18, 20]
-
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
       - name: Install pnpm
         uses: pnpm/action-setup@v4
 
-      - name: Setup Node.js ${{ matrix.node-version }}
+      - name: Setup Node.js 20
         uses: actions/setup-node@v4
         with:
-          node-version: ${{ matrix.node-version }}
+          node-version: 20
           cache: pnpm
 
       - name: Install dependencies