Closes https://github.com/zed-industries/zed/issues/52346

Given the parallel nature of subagents calls, it's possible that there
is a subagent way out of view that's waiting for the user to give
permissions. Right now, it's kind of hard to know this and you may think
something wrong is happening given the thread generation isn't making
any progress.

This PR adds an "activity bar" to the thread view that displays
subagents on a "waiting for confirmation" status. We display the
subagent's summary label as well as allow clicking on it to quickly
scrolling to that subagent.

<img width="500" height="1100" alt="Screenshot 2026-03-25 at 10  09@2x"
src="https://github.com/user-attachments/assets/17d2d398-25fc-4660-85c2-bf66ec98c1f1"
/>

Release Notes:

- Agent: Improved the experience of interacting with subagents waiting
for confirmation.