agent: Remove priority from update_plan (#52320)

## Context

We don't surface this in the UI, so removing it to make the input
smaller.

## 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:

- N/A
4b60e639bcBen Brandt committed on 3/24/2026, 1:16:10 PM· committed by GitHubparenteeafae3
2 files changedLine totals unavailable