ci: use CURSOR_PREFERRED_MODEL secret instead of hardcoded model

Replace hardcoded --model opus-4.5 with --model ${{ secrets.CURSOR_PREFERRED_MODEL }}
in the cursor-agent CLI invocation, allowing the model to be configured via
GitHub Actions secrets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: masnwilliams

.github/workflows/update-cli-coverage.yml

@@ -411,4 +411,4 @@ jobs:
           - Streaming methods may have different CLI implementations (e.g., follow flags)
           - Even if no coverage gaps are found, still create a PR for the SDK version bump
           - Ensure code compiles before pushing
-          " --model opus-4.5 --force --output-format=text
+          " --model ${{ secrets.CURSOR_PREFERRED_MODEL }} --force --output-format=text