Feature: simplify multi-goal and multi-agent workflows

[davidzhao]

Certain workflows are more complex to orchestrate, often involving multiple states transitions and prompts.

While it's currently possible to orchestrate these workflows, it's quite complex to do so. we need to make it simple to transition users between agents (or states) within the same session.

Ideally there's also the flexibility of wiring inputs and outputs of the agents manually.

related to: #954