rt: Reparent the agents context to the root.
rt: Reparent the agents context to the root.

We already reparent the contexts servers use for publishing so they can
finish unmounting after the runtime starts to shutdown.  They also
need to be able to talk to the agent to make that RPC and the agent
uses a context it derives at runtime initialization.

Change-Id: Ifffbc623bb0f8d9e3ecd9ad87f0ec8823913d88c

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref