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_

首先定位现有的 /usage 命令，并跟踪它如何获取和显示每个会话的 AIC 使用量。确定月度限额、使用量、百分比和重置时间是否可供 CLI 使用，然后让选定的命令显示全部四个值，并根据提议的示例验证输出。

由索引模型根据 Issue 内容生成。