Add a Skill Builder

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

There is no skill builder that works the same way the agent builder works.

### Proposed solution

A new parameter to `/skill`:
- `/skill create`

Copilot then asks a few questions, and creates the optimal Skill file in .github/skills

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先追蹤現有的 agent 建立流程和 `/skill` 命令進入點。定義建立 skill 所需的問題，並確認使用 `/skill create` 時，完成的流程會在 `.github/skills` 下產生適當的檔案。

由索引模型根據 Issue 內容生成。