Add _create_plus_client_hook to the skill registry, mirroring the
existing pattern in agent_utils for from_repository agents.
When set (by the Studio Runner or similar host), download_skill() uses
the hook-provided PlusAPI client instead of the default unauthenticated
one. In non-interactive mode, the SkillNotCachedError is only raised
when no hook is available — allowing deployed environments to
auto-download skills on first use.
Co-authored-by: João Moura <joaomdmoura@gmail.com>
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