Granular Permissions for Repository Secrets and Variables

### Value Prop

You can now assign permissions for managing repository secrets and variables without giving full write access to your repository. This makes it easy to control who can create, update, or delete sensitive credentials, so your team can collaborate securely while keeping important information protected.

### Expected Outcome

By separating secrets and variables management from general write access, organizations can better enforce least-privilege access and reduce the risk of accidental or unauthorized changes to sensitive data. This helps teams maintain strong security and compliance standards without disrupting their regular development workflows.

Issue 中没有确定实现文件、测试或入口点。首先明确权限模型，以及受影响的 repository secrets 和 variables 工作流；完成的标准是用户可以独立于常规的 repository 写入权限管理这些资源。

由索引模型根据 Issue 内容生成。