Energy app is now watchface!

We're now a watchface!

NL Energy v1.0.0

docs: clear install instructions for end users

- energy: real description, NL-only caveat, on-screen/controls guide,
  show preview, fix the platform claim (emery-only)
- root: add Install section — prebuilt energy.pbw via Releases vs
  build-from-source; voice-notes is build-your-own (bridge secret
  compiles in, so no prebuilt is shipped)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>