feat: add CI matrix testing for Node.js 20 and 22

Adds a node-compat job to the CI workflow that tests pnpm install
against both Node.js 20 (LTS) and 22 (current LTS) using corepack.
Both matrix entries must pass for CI to be green.

Closes #16

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Paul Mulligan

.github/workflows/ci.yml

@@ -178,6 +178,29 @@ jobs:
           echo "$errors template validation failures"
           exit $errors
 
+  node-compat:
+    name: Node.js ${{ matrix.node-version }} Compatibility
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    strategy:
+      matrix:
+        node-version: [20, 22]
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+
+      - run: corepack enable
+      - run: pnpm install
+
+      - name: Verify installation
+        run: |
+          echo "Node.js version: $(node --version)"
+          echo "pnpm version: $(pnpm --version)"
+          pnpm ls --depth 0
+
   check-links:
     name: Check Markdown Links
     runs-on: ubuntu-latest