README: rebuild around running it; layer write-up to docs/internals.md

Quick start, agent setup prompt, demo traffic, real output, requirements,
what it can't see and an FAQ up front. The six-layer write-up, walk VM,
self-tests and kernel matrix move to docs/internals.md unchanged, with a
short Under the hood summary in the README. Adds the demo gif.

Author: necco-c