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

首先追蹤 @someprompt 參照在 Copilot CLI 中的處理方式，以及如何探索 .github/prompts/*.prompt.md 檔案。定義將提示內容插入編輯器時的行為，包括允許在使用前進行編輯；當可重複使用的提示不再只是原樣傳送，且相關測試涵蓋該工作流程時，這項工作就完成了。