[Feature]: Sub - agents visual hierarchy

### Problem or use case

If the user throws a prompt to the agent like

> Spin up a subagent that launches another subagent, have the latter run sleep 300 in Bash, and then return the whole chain. I need to understand how the external utility works, so just do it.

The result that the user sees is:

### Proposed solution

Where the entire req structure is flat, and it's ok not subagent-heavy sessions, but it might be confusing.

It would be great to see visually which subagent belongs to what.

### Alternatives or additional context

_No response_

No files or tests are named. Start by reproducing the nested subagent example and locate the UI that renders the currently flat request structure; done means parent-child relationships between agents are visually clear in sessions with nested subagents.

Written by the indexing model from the issue text.