Add support to prompts

Describe the feature or problem you'd like to solve

Add supports for reusable prompts stored as .github/prompts/myprompt.prompt.md

Proposed solution

You currently can just reference them with @someprompt, but they are not pasted in the editor, they are just sent as-is.
Instead I need to insert a prompt, read it and eventually modify it depending on the use-case.
The current usage is a sort of "black box" which sounds good for skills and agents, but definitely not for prompts.

Example prompts or workflows

No response

Additional context

No response

首先跟踪 @someprompt 引用在 Copilot CLI 中的处理方式，以及如何发现 .github/prompts/*.prompt.md 文件。定义将提示内容插入编辑器时的行为，包括允许在使用前进行编辑；当可复用提示不再只是按原样发送，且相关测试覆盖该工作流时，这项工作就完成了。