GitHub CLI: Image upload and attach for issues, PRs, and comments [GA]

### Value Prop
You can now attach local images and videos directly to issues, pull requests, and comments using the GitHub CLI. This works across `gh issue create`, `gh issue comment`, `gh pr create`, `gh pr comment`, and `gh pr edit`.

### Expected Outcome
Developers and teams using GitHub CLI in automated or agentic workflows gain a reliable way to include visual evidence like screenshots, recordings, and diagrams alongside their work. This closes a long-standing gap that made it difficult to provide proof of work or communicate changes clearly through the CLI alone. The result is a more complete CLI experience that fits naturally into both human and agentic development flows.

该 issue 指出了包括 gh issue create、gh issue comment、gh pr create、gh pr comment 和 gh pr edit 在内的 GitHub CLI 命令，但没有列出任何仓库文件或测试。首先定位这些命令的实现以及现有的附件处理方式。完成标准是：在列出的每个工作流中，都能可靠地附加本地图像和视频。

由索引模型根据 Issue 内容生成。