Delete images from context

### Describe the feature or problem you'd like to solve

images are good for comunicating with copilot, but once used they remain using context space

### Proposed solution

Keep the context active longer.

### Example prompts or workflows

_No response_

### Additional context

_No response_

未指定任何文件、测试或入口点。首先明确目标是使用后删除图像，还是延长其上下文的生命周期；然后定位 CLI 的图像上下文处理，并定义测试，以展示所选行为并确认上下文仍能按预期使用。

由索引模型根据 Issue 内容生成。