Follow up to https://github.com/zed-industries/zed/pull/52730. 

This PR adds a boolean setting `limit_content_width` in the Agent Panel
settings that allows turning off the content max-width entirely, which
was added for better readability. We had a handful of requests for it,
so it feels fair.

<img width="500" alt="Screenshot 2026-04-20 at 8  57@2x"
src="https://github.com/user-attachments/assets/d2540b35-3fa8-4424-895d-dc499ac4839c"
/>

Release Notes:

- Agent: Added a new `limit_content_width` setting in the agent panel
that allows turning off the content max-width limit.