Clarify command-line installation path #8

Backplane publishes nightly prerelease artifacts on GitHub Releases, including Linux AppImages and Android APKs. However, several guides direct command-line hosts to run npx @backplane/cli@latest, while @backplane/cli is not currently published on npm.

This leaves command-line setup, background-service, and update instructions unusable as written. The documentation should distinguish the nightly desktop/mobile installation path from source-built command-line-host usage until a published CLI is available.