Notification when model context was read by model

[ochafik]

ui/update-model-context (#125) leaves context for the model to pick up at some point (next user turn at the latest), leaving the app in the dark as to what to put in there (repeat everything previously said, or add new stuff only? only the last version of the context is passed to the model) and as to whether it's been read or not.

Here are two potential ways to fix this:

• onmodelcontextread: new notification that lets the app know when the context "inbox" was read
• Add a wait?: boolean to ui/update-model-context so its result is suspended until the context was read by the model (success), or until a concurrent model context update was sent (failed w/ appropriate json rpc error code)

cc/ @idosal