Every subagent stays under the chat that started it
Finished subagents stay nested under their chat in the sidebar, at any depth, and a click on the chat's name folds or unfolds them.
Subagents used to leave the sidebar once they finished. Now each one stays indented under the chat that started it, and a subagent's own subagents sit one step further in.
A chat whose subagents have all finished starts folded, with a dot and a count beside its name. Click the name to unfold it, click again to fold it. The arrow beside the name and the arrow keys do the same.
Your choice is kept for each chat after the window reloads.