Sponsoring a tool, source or prompt the agent's owner can't use now takes
owning it or having edit access to it; use access alone no longer extends it
to the agent's audience. A save that would make the caller a new sponsor is
refused with 409 sponsor_confirmation_required (the resources and the
agent's audience) until it is retried with confirm_sponsor listing them.
When a sponsor loses access, the resource stops instead of passing to
whoever saves next; another editor takes it over only by confirming.
Workflows follow the same rules. sponsor_details now reports each
sponsorship's state, the reason it stopped, and whether the reader can
take it over.
arc53-machine