Cross harness orchestration

[CalumH97]

Unsure if this is me being stupid but is it possible to for example get Fable spawning Sol subagents both within their respective harnesses so I can keep using their own subscriptions whilst retaining the t3 interface and not using CLI for the sub agents?

[juliusmarminge]

Closing as a duplicate of #6741, which tracks cross-provider delegation and orchestration inside T3 Code.