Docs: document vg install --compress / vg savings token compression

[vibgrate-team]

Context

vg install <agent> --compress and vg savings are part of the local-first token-compression path (listener on the machine, originals stay local). New contributors and CI authors need a short, accurate public doc of what compress does, what it never uploads, and how to read vg savings windows (today / 7d / 30d).

Guardrails: local-first, no secrets in examples, deterministic wording for flags/exits.

What to do

1. Add a focused section (README and/or a small docs page that the public site can deep-link) covering:
   • vg install <agent> --compress vs vg serve --compress
   • how undo/vg uninstall behaves
   • vg savings output fields and time windows
   • honest caveats (what is estimated $, what is measured tokens)
2. Prefer vg in all examples; no API keys or cloud credentials in sample commands.
3. Link the live CLI simulator / help pages where they already exist.

Acceptance

• A newcomer can enable compression, confirm the listener is local, and interpret vg savings without reading source
• Examples use vg, stay local-first, and never embed secrets
• DCO sign-off on commits (git commit -s); pnpm test / lint / typecheck green if code touches docs tooling