I'm trying very hard to understand the Node ecosystem and how npm and its packages work together and I find it very odd that
npm grunt init
npm init does is give you prompts (see below) to create a package.json, there's nothing grunt-related that comes with it.
name: (test) version: (0.0.0) description: entry point: (index.js) test command: git repository: keywords: author: license: (BSD)
Grunt can certainly (and probably should be) installed with npm but it's definitely not part of initializing your package.json.
You can read more about package.json here.