ci: remove pnpm version to defer to packageManager

pnpm/action-setup@v4 errors when both action input "version" and
package.json "packageManager" are set. Drop the action input so
package.json remains the single source of truth.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -22,8 +22,6 @@ jobs:
 
       - name: Setup pnpm
         uses: pnpm/action-setup@v4
-        with:
-          version: 10
 
       - name: Setup Node.js
         uses: actions/setup-node@v4