ci: add typecheck workflow to validate TypeScript declarations

Runs npm run check:types on push and PR to main, matching the existing
lint/test/build workflow pattern.

Author: Mearman

.github/workflows/typecheck.yml

@@ -0,0 +1,22 @@
+name: Typecheck CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  typecheck:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Node.js
+      uses: actions/setup-node@v4
+      with:
+        node-version: 22
+        cache: 'npm'
+    - run: npm install
+    - run: npm run check:types