[feat] Display subagent activity in a new pane #5077

Describe the feature

When one agent delegates work to a subagent, I need to navigate to that subagent to see what it is doing. It would be handy to have a way to open the subagent in a pane/window to the right of the main agent, so you can see both simultaneously. Subsequent subagents could be displayed in a pane under the other subagent.

Cmux seems to support something like this (I think, haven't tried it), see https://cmux.com/docs/agent-integrations/claude-code-teams