AI Credits Near-Limit Warning

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

Visual Studio 2026 Professional IDE warns users in chats sessions when they are nearing their AI Credits limit for their GitHub Copilot subscription. It would be great to have the GitHub Copilot CLI have this feature parity.

### Proposed solution

at the beginning of each new chat session for GitHub Copilot CLI, if the session is valid (i.e. successful user login/authentication), and the user has spent more than 80% of their total AI credit limit according to their GitHub Copilot subscription, then show a brief warning message at the chat session start, to notify them, with an optional URL to visit to increase their limit.

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先跟踪 GitHub Copilot CLI 在开始新聊天会话时的入口点，以及成功登录/身份验证的路径。确定如何访问用户 GitHub Copilot 订阅的信用额度使用量和总限额。完成的标准是：经过身份验证的会话在使用量超过 80% 时显示一条简短警告，并可选择链接到一个可以提高限额的页面。

由索引模型根据 Issue 内容生成。