README: the prebuilt ghcr image is the default install

Quick start now says what is true since e5554bb: the image is prebuilt and
pushed on every commit with the patch application + verify.sh build gate,
so the only local work is the model download/requantization. The Setup
section is reframed as the manual venv path for hacking on the stack or
running bare-metal. (Package flipped public; anonymous pulls verified.)