Make Steer and Queue visible choices before sending

### Feature summary

Show a visible choice between steering the current work and queueing the next message.

### What problem are you trying to solve?

When I wanted to send another message while Copilot was working, I could not tell whether it would change the current task or wait until it finished. I ended up not sending it because I did not want to disturb the work in progress.

I only later learned that Enter steers and Ctrl+Enter queues. Queueing already exists, but this choice is difficult to discover unless you already know the shortcut.

### Proposed solution

While Copilot is working, show both actions in the Send button or its dropdown:

- Steer current work
- Add to queue

The existing shortcuts can remain as they are.

VS Code already exposes the same choices while a request is running:
https://code.visualstudio.com/docs/chat/chat-overview#_send-messages-while-a-request-is-running

### Workflow impact

I could capture next steps immediately without accidentally redirecting the current task or having to remember to return later.

### Installation context

GitHub Copilot app 1.0.23 on Windows x64, OS build 26200.8655, in the /chat view.

### Additional context

Related: #27 introduced queueing, #275 explains the current shortcut, and #1757 shows that queueing can still be difficult to discover.

This request is specifically about making the existing choice visible before sending.

Start by inspecting the /chat view and the existing queueing behavior, then compare the requested interaction with the linked VS Code reference. Done means both “Steer current work” and “Add to queue” are visible while Copilot is working, while the existing keyboard shortcuts continue to work.

索引モデルが issue の本文から書いたものです。