ci: enable pnpm cache

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

Author: silverwind

.github/workflows/ci.yaml

@@ -13,6 +13,7 @@ jobs:
       - uses: pnpm/action-setup@v4
       - uses: actions/setup-node@v4
         with:
+          cache: pnpm
           node-version: latest
       - uses: oven-sh/setup-bun@v1
         with: