refactor: remove setup feature from build exports and config

Removes the `./setup` export entry from `package.json` and the
corresponding `src/features/setup/index.js` entry point from
`tsup.config.js`, eliminating the standalone setup feature module
from the public API and build output.

src/features/setup/index.js

@@ -1,6 +0,0 @@
-/**
- * Zen Setup Module
- * Utilities for setting up Zen in a Next.js project
- */
-
-export { setupZen } from './cli.js';