fix: remove packageManager field and fix Windows npm spawn

- Remove packageManager from package.json to avoid version conflict
  when the action tests against itself (uses: ./)
- Use shell: true on Windows so spawn can find npm.cmd

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

package.json

@@ -1,6 +1,5 @@
 {
   "private": true,
-  "packageManager": "pnpm@10.32.1",
   "scripts": {
     "build:bundle": "esbuild src/index.ts --bundle --platform=node --target=node24 --format=cjs --minify --outfile=dist/index.js --loader:.json=json",
     "build": "pnpm run build:bundle",