Plugin subagent dispatch used a hardcoded synthetic client carrying
operator.admin, operator.approvals, and operator.pairing for all
runtime.subagent.* calls. Plugin HTTP routes with auth:"plugin" require
no gateway auth by design, so an unauthenticated external request could
drive admin-only gateway methods (sessions.delete, agent.run) through
the subagent runtime.
Propagate the real gateway client into the plugin runtime request scope
when one is available. Plugin HTTP routes now run inside a scoped
runtime client: auth:"plugin" routes receive a non-admin synthetic
operator.write client; gateway-authenticated routes retain admin-capable
scopes. The security boundary is enforced at the HTTP handler level.
Fixes GHSA-xw77-45gv-p728
