fix(build): make cleanup script cross-platform

[9426Haruka]

What

Replace the Unix-only cleanup command (
m -rf ... && rm -rf ...) in the root package.json with a small Node script that reads the workspace list and removes dist / .output directories using
ode:fs. No new dependencies.

This keeps
pm run cleanup portable across Windows, macOS, and Linux.

Type

• Breaking change
• Bug fix
• Feature / Improvement
• Refactor / Chores
• Documentation / Website / Demo / Testing

Testing

• 
  pm run ci\ passes
• Tested in modern browsers
• Types/doc added

Verified locally on Windows:

• Created fake artifacts in \packages/core/dist\ and \packages/website/.output, ran
  pm run cleanup, and both directories were removed.
• 
  px prettier --check scripts/cleanup.js package.json\ passes.
• 
  px eslint scripts/cleanup.js\ passes.

Requirements / 要求

• I have read and follow the Code of Conduct and Contributing Guide. / 我已阅读并遵守行为准则。
• This PR is NOT generated by a bot or AI agent acting autonomously. I have authored or meaningfully reviewed every change. / 此 PR 不是由 bot 或 AI 自主生成的，我已亲自编写或充分审查了每一处变更。