fix: remove packageManager field, restore pnpm to CI matrix

packageManager field conflicted with siguici/setup-js passing
version:latest to pnpm/action-setup. Removing it lets the CI
matrix test all three PM variants (auto-detect, pnpm, yarn).

Author: thejackshelton

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
         os: [ubuntu-latest, macos-latest, windows-latest]
         experimental: [false]
         runtime: [node, deno, bun]
-        pm: ["", yarn]
+        pm: ["", pnpm, yarn]
 
     steps:
       - uses: actions/checkout@v4

package.json

@@ -53,6 +53,5 @@
   },
   "engines": {
     "node": ">=24.0.0"
-  },
-  "packageManager": "pnpm@10.25.0"
+  }
 }