Write files into a Vercel Sandbox and download build artifacts back to the local filesystem using the JS/TS SDK file APIs.

Documented steps

1. Create the sandbox: `const sandbox = await Sandbox.create()`.
2. Create a directory if needed with `await sandbox.mkDir('src')` (paths are relative to `/vercel/sandbox` unless absolute). Docs: https://vercel.com/docs/sandbox/sdk-reference
3. Write one or more files in a single call: `await sandbox.writeFiles([{ path: 'src/build-artifact.js', content: Buffer.from('...') }])`. Batch related files into one call to reduce round trips. Use the optional `mode` field (e.g. `0o755`) to set Unix permissions for executable scripts.
4. Run the code that produces output: `const result = await sandbox.runCommand('node', ['src/build-artifact.js']); if (result.exitCode !== 0) throw new Error(await result.stderr());`.
5. Download the resulting artifact: `await sandbox.downloadFile({ path: 'dist/output.txt' }, { path: './artifacts/dist/output.txt' }, { mkdirRecursive: true })`. Use `sandbox.readFile()` (returns a `ReadableStream | null`) or `sandbox.readFileToBuffer()` (returns `Buffer | null`) to pull contents into memory instead of to disk.
6. Call `await sandbox.stop()` when done.