ci: use CURSOR_PREFERRED_MODEL secret instead of hardcoded model (#138)

## Summary

Replaces the hardcoded `--model opus-4.5` in the cursor-agent CLI
invocation in `.github/workflows/fix-ci.yaml` with `--model ${{
secrets.CURSOR_PREFERRED_MODEL }}`, allowing the model to be configured
via GitHub Actions secrets instead of requiring a code change.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk workflow-only change that just swaps a hardcoded model name
for a configurable secret; main risk is misconfiguration if the secret
is unset or invalid.
> 
> **Overview**
> Updates the `Fix CI Failures` GitHub Actions workflow to stop
hardcoding the `cursor-agent` `--model` value and instead read it from
`${{ secrets.CURSOR_PREFERRED_MODEL }}` for configurable model selection
without code changes.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
c61cac1. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: masnwilliams

.github/workflows/fix-ci.yaml

@@ -102,4 +102,4 @@ jobs:
           - Avoid duplicate comments - update existing bot comments
           - If no actionable fix is possible, make no changes and post no comment
           - PR comments must ONLY include the PR creation link, no manual merge instructions
-          " --model opus-4.5 --force --output-format=text
+          " --model ${{ secrets.CURSOR_PREFERRED_MODEL }} --force --output-format=text