## Summary

Closes #55958

When no conversation has started yet, the agent panel's message editor
uses `EditorMode::Full`. Wrap guides are only drawn in full mode, so a
user's `wrap_guides` setting (for example `"wrap_guides": [100]`) showed
a vertical line across the empty agent panel. The line does nothing
useful there.

The message editor already turns off indent guides. This PR turns off
wrap guides in the same place, the same way the inline assistant's
prompt editor already does (`inline_assistant.rs`).

## Testing

1. Add `"wrap_guides": [50]` to `settings.json`.
2. Open the agent panel before starting a conversation, and make it wide
enough to reach column 50.
3. Before this change a vertical wrap guide shows in the empty panel.
After it, none shows.

`cargo clippy -p agent_ui --all-targets -- --deny warnings` and `cargo
fmt --check` pass.

## Self-review

- [ ] I've reviewed my diff for quality, security, reliability, and
performance.
- [ ] UI changes follow the
[checklist](https://zed.dev/docs/development/ui-checklist).
- [ ] Tests cover the new or changed behavior.

Release Notes:

- Fixed wrap guides showing in the agent panel before a conversation
starts.