Display monthly AIC quota and usage in Copilot CLI (like IDE plugins do)

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

Currently, the Copilot CLI only shows per-session AIC usage via the /usage command. However, IDE plugins (like IntelliJ) display monthly quota information (e.g., "6219/12000 used, 52% of monthly limit").

### Proposed solution

Add a command (e.g., /quota or extend /usage) that displays:

- Monthly AIC limit (e.g., 12,000)
- AIC used this month (e.g., 6,219)
- Percentage used (e.g., 52%)
- Days until reset (e.g., "Resets in 6 days")

### Example prompts or workflows

_No response_

### Additional context

_No response_

先找到現有的 /usage 命令，並追蹤它如何取得及顯示每個工作階段的 AIC 使用量。確認每月上限、使用量、百分比和重設時間是否可供 CLI 使用，然後讓選定的命令顯示全部四個值，並根據提議的範例驗證輸出。

由索引模型根據 Issue 內容生成。