feature request: A dedicated pane to track running sub-agents

Sub-agents are a great addition in v4. Right now the only way to see what a sub-agent is doing is inside the chat itself. With multiple sub-agents running, the chat turns into interleaved status lines, and it's hard to tell which agent is doing what, or how many are even running.

A dedicated pane (like the file explorer or outline panes) listing every sub-agent would fix this: name, status (running/done/failed), and what it's currently doing. Clicking an entry could jump to that agent's part of the chat.

I understand this is a bigger change than a small tweak. But agent mode is now the core of v4, and agents running in parallel need somewhere to be watched besides scrolling through chat.