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>

src/install-pnpm/run.ts

@@ -74,6 +74,7 @@ function runCommand(cmd: string, args: string[], opts: { cwd: string }): Promise
     const cp = spawn(cmd, args, {
       cwd: opts.cwd,
       stdio: ['pipe', 'inherit', 'inherit'],
+      shell: process.platform === 'win32',
     })
     cp.on('error', reject)
     cp.on('close', resolve)