Library root supervisor for the firmware-update flow.

The single public entry point the host imports. Hosting application child-specs this module (not Updater directly); at library extraction time the host swaps NervesGithubUpdater.Supervisor → NervesGithubUpdater.Supervisor in one place.

Accepts the same opts as NervesGithubUpdater.Updater.start_link/1 and passes them through to a single Updater child.