The sandbox's context builder was a near-copy of the Node runtime's. Both
now come from createPluginContext in @yaakapp-internal/lib, with each
runtime supplying only a transport.
The two places hosts genuinely differ are optional transport methods:
`stream` (a window reporting navigation until it closes) and `form` (a
prompt that re-renders as values change). The sandbox has neither, so
openUrl refuses and a form is drawn once from its defaults.