Package update implements self-update for the supermodel binary.
It fetches the latest release from GitHub, downloads the archive for the
current OS/arch, extracts the binary, and atomically replaces the running
executable.
Run checks for a newer release and, if found, downloads and installs it.
Returns (true, nil) when an update was installed, (false, nil) when already
up to date, and (false, err) on failure.