ci: drop npm cache and use npm install — package-lock.json is gitignored

setup-node's cache:npm and npm ci both require a committed lockfile,
which this repo intentionally gitignores.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jack-arturo

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 22
-          cache: npm
-      - run: npm ci
+      # package-lock.json is gitignored in this repo, so no npm ci / cache
+      - run: npm install
       - run: npx tsc --noEmit
       - run: npm run build