## Context

Ensure subagent threads build requests with the Subagent intent instead
of UserPrompt. This allows us to properly
attribute this as a tool call for certain providers instead of a user
request.

## Self-Review Checklist

<!-- Check before requesting review: -->
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- copilot_chat: Fix subagent requests being marked as user requests.