chore: declare supported Node engine range (>=18)

[rmyndharis]

Summary

The published package has no engines.node, so consumers get no signal about the minimum supported Node version. The CLI/library uses modern Node features — ESM with import.meta.url + fileURLToPath, an exports map, and async iteration over process.stdin — so a floor is appropriate.

Declares engines.node: ">=18.0.0". This is advisory by default (npm warns below it; it does not hard-block unless the consumer sets engine-strict), so it is non-breaking.

Testing

Metadata only. package.json validated as parseable; node/bun resolution unaffected.

[rajpratham1]

This is a small but useful maintenance change. Declaring the supported Node.js engine in package.json helps package managers warn users when they're using an unsupported runtime and makes the project's runtime requirements explicit. Since it doesn't change application logic or dependencies, it's a safe improvement.