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_

Start by tracing the GitHub Copilot CLI entry point for the beginning of a new chat session and the successful login/authentication path. Determine how the user's GitHub Copilot subscription credit usage and total limit can be accessed. Done means an authenticated session shows a brief warning above 80% usage, optionally linking to a page where the limit can be increased.

Written by the indexing model from the issue text.