SEP: Define state behavior

[idosal]

App-related state has multiple sources (e.g., tool input/output). Some sources may change during the App's lifecycle (e.g., follow-up tool outputs).

We must define the behavior around state updates and rehydration.

[idosal]

For the MVP, it'll remain simple. Tool input/output is the only persistent state, and it's immutable. Hosts may choose to render apps in different layouts (e.g., in a side panel or on top of each other when the app is full screen), but each invocation would result in a new UI instance.

After the MVP, it's likely to be expanded with capabilities such as -

1. Persistent mutable state
2. Persistent containers (e.g., session ID mechanism)