chore: replace mkdirp with native fs.mkdirSync

[sarahdayan]

Summary

Removes the mkdirp dependency by using Node.js native fs.mkdirSync with the recursive option.

Why

• fs.mkdirSync(path, { recursive: true }) has been available since Node.js 10.12.0
• Ship.js requires Node.js 20+, so this native API is guaranteed to be available
• Reduces external dependencies

Changes

• Replace mkdirpSync calls with fs.mkdirSync(path, { recursive: true }) in:
  • addCircleCIConfig.js
  • addGitHubActions.js
• Remove mkdirp from dependencies

[Haroenv]

actually CI is failing, seems unrelated but not sure what exactly happens

[sarahdayan]

@Haroenv This was some conflict caused by the upstream Vitest migration. It's good now.