dispatch: suggest --repos only where the flag is accepted

resolveOriginRepoSlug is shared by cloud and local dispatch, but its errors told the user to pass --repos, which --local refuses. A local dispatch in a checkout whose origin Entire cannot address pointed the user at a flag that then failed with "--repos cannot be used with --local".

The shared helper now names no flag; the cloud caller adds the --repos hint when it wraps the error.