Copy a readable reference table into your organization under a new slug.

The source may be a Nebulock-curated table -- cloning is how a tenant takes a curated
list and starts editing it. The clone always lands in your organization, so the new
slug is validated against your ownership: a tenant cannot clone nebulock_browsers
and keep the reserved prefix.

visibility is optional and follows the same rule as create: public from any
organization, private from Nebulock only. It is never inherited from the source --
cloning a published curated table gives you a table of your own, and carrying its
publish state over would be a claim about the copy that nobody made. Omit it for null.