Publish @jetti/cli to npm

The install CLI is currently read in-repo. Ship it as a real npm package so users can run `npx @jetti/cli install ...` without cloning anything.

Acceptance criteria

Refactor `scripts/jetti-install.mjs` into a CLI entrypoint with subcommands (`install`, eventually `apply`)
`npx @jetti/cli install` works against a fresh Vite app
`package.json` already scaffolds the `bin` entry (jetti-install) — extend to a single `jetti` bin with subcommands
Tag and publish v0.1.0
Update README quickstart to use `npx @jetti/cli install` once published