This PR:
- Moves the "remove workspace" option into the context menu, so that
it's not swapping places with the + button in some circumstances. This
matches codexs behavior. Another solution to the problem could be having
the add button remain even when the "new thread" option exists.
- Adds a "go to workspace" button to the header

## 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

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>