Add GitHub Check name input

[thebiglabasky]

Summary

• Add a check-name input for GitHub Check reporting.
• Export CHECKLY_GITHUB_CHECK_NAME when detached GitHub Check reporting is available.
• Include the field in preflight metadata and dry-run output.

Testing

• ./scripts/test.sh

Notes

• Custom names require a CLI/backend version that forwards and consumes repoInfo.github.checkName; otherwise the backend default remains Checkly.

[thebiglabasky]

Superseded by #6, which includes the custom GitHub Check name/context metadata changes plus the CLI 8.15.0 compatibility guard and expanded CI harness.