chore: replace fs-extra with native node:fs

[roli-lpci]

Summary

• Replace fs-extra dependency with native node:fs and node:fs/promises equivalents
• Node.js has supported fs.rm, fs.cp, fs.mkdir({ recursive: true }), and other needed APIs natively since v16
• Reduces the dependency tree

Changes

• lib/util.js — replaced fs.ensureDir with fsp.mkdir({recursive: true}), fs.copy with fsp.copyFile
• lib/git.js — replaced fs.exists with fs.existsSync, fs.mkdirp with fs.mkdirSync({recursive: true})
• lib/index.js — replaced fs.removeSync with fs.rmSync, fs.stat with fsp.stat, fs.createFile with fsp.writeFile, fs.writeFile with fsp.writeFile
• test/helper.js — replaced fs.copy with fs.cpSync
• package.json — removed fs-extra from dependencies

Testing

• All 32 existing tests pass
• Linter passes clean

Part of the e18e ecosystem cleanup initiative.