fix(ci): restore pnpm cache now that lockfile is on main

pnpm-lock.yaml landed with PR #15, so setup-node can cache pnpm deps
again. This speeds up CI by avoiding full re-installs.

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

.github/workflows/ci.yml

@@ -18,6 +18,7 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
+          cache: pnpm
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
@@ -39,6 +40,7 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
+          cache: pnpm
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
@@ -58,6 +60,7 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
+          cache: pnpm
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile