Yoi instance Plugin template

This template targets yoi:plugin/instance@1.0.0. The host creates one PluginInstance for the package; Tool, Service, and Ingress surfaces share that instance state while each surface keeps separate permissions/grants.

Tools still run only through ordinary model/user-initiated Tool calls. Ingress handlers receive bounded typed untrusted events and must return explicit JSON for host-mediated visible/durable paths.