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_

No files, tests, or entry points are named. First clarify whether the goal is to delete images after use or extend their context lifetime, then locate the CLI's image-context handling and define tests that show the selected behavior and confirm context remains usable as intended.

索引モデルが issue の本文から書いたものです。