New per-field unavailability message, used for run_in_background

An internal helper that used to only show a "Checking GitHub CLI installation..." spinner has been turned into a general message generator producing lines like "{field} is not available on {target}." It has a special case for run_in_background: when it's unavailable, the message explains that the calling session has no way to poll a background task there, and suggests running it in the foreground or in the session's own environment instead.