Add Node.js version gate and fix ESM path resolution

[Copilot]

cli.js uses top-level await and native ESM, requiring Node ≥ 18. There was no runtime guard against older versions, and src/utils/fs.js had a silent bug (import.meta.dirname is non-standard and returns undefined).

Changes

• src/utils/node-version.js — new utility; checks process.version against the Node 18 minimum and exits with a clear upgrade message if not met
• src/cli.js — calls checkNodeVersion() before any setup logic
• package.json — adds "engines": { "node": ">=18.0.0" } for tooling-level enforcement
• src/utils/fs.js — fixes projectRoot() to use dirname(fileURLToPath(import.meta.url)) instead of import.meta.dirname

// src/utils/node-version.js
export function checkNodeVersion() {
  const major = parseInt(process.version.replace(/^v/, "").split(".")[0], 10);
  if (major < 18) {
    console.error(
      `suitup requires Node.js 18 or later for native ESM support.\n` +
      `You are running Node.js ${process.version}.\n` +
      `Please upgrade: https://nodejs.org/en/download`
    );
    process.exit(1);
  }
}

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