Sub instances keep running after the main instance is stopped with Esc

[plgodin]

When stopping the main agent with the Escape key, I can see that sub agents continue processing and consume tokens, even though their final output gets ignored by the main agent, leading to out-of-sync context as the sub agents think they replied to the main agent, but the main agent doesn't have that context.

I feel like interruptions on the main agent should interrupt sub agents too.

[parruda]

We need the fast-mcp gem to implement cancelations to fix this.