A MoA preset whose reference or aggregator slot points at the moa virtual
provider creates a recursive MoA tree. The runtime guards in moa_loop.py only
surface this mid-turn (references silently skipped, aggregator raises). Reject
it at the config chokepoint (_clean_slot) so it can never be saved, and hide it
from the desktop/dashboard slot pickers so it isn't offered as a dead choice.
