docs: friendly README + 7 linked sub-docs

Root README.md rewritten as concise, inviting overview.
New docs in docs/:
  install.md       — prerequisites, setup, start scripts, config
  architecture.md  — stack diagram, module responsibilities, data flow
  virtualbox.md    — VM lifecycle, snapshots, networking, console, ISO
  hyper-v.md       — requirements, limitations compared to VBox
  sandbox.md       — launch, dev tools, airgap, fleet install
  usage-cases.md   — 7 real workflows with example prompts

Author: User