docs: remove GitHub Models references from README

[anticomputer]

GitHub Models is no longer available to new customers.

Updates the README to:

• Point at GitHub Copilot instead of GitHub Models for token entitlement
• Change the example AI_API_ENDPOINT from models.github.ai/inference to api.githubcopilot.com

The github-models provider remains functional in capi.py for existing users; this only removes it from documentation.

[Copilot]

Pull request overview

This PR updates the project’s README configuration guidance to remove references to GitHub Models (no longer available to new customers) and align the documented defaults with GitHub Copilot.

Changes:

• Updates the README to reference GitHub Copilot (instead of GitHub Models) for AI_API_TOKEN entitlement.
• Updates the example AI_API_ENDPOINT from https://models.github.ai/inference to https://api.githubcopilot.com.

Show a summary per file

File	Description
README.md	Updates configuration docs to point to GitHub Copilot and the Copilot API endpoint instead of GitHub Models.

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README.md:212

• AI_API_TOKEN is described as a “GitHub token”, but this env var is used as the bearer token for whichever AI_API_ENDPOINT is configured (the code supports non-GitHub providers like OpenAI as well). This wording (and the <your_github_token> placeholder) can mislead users who point AI_API_ENDPOINT at a non-GitHub API.

Provide a GitHub token for an account that is entitled to use [GitHub Copilot](https://github.com/features/copilot) via the `AI_API_TOKEN` environment variable. Further configuration is use case dependent, i.e. pending which MCP servers you'd like to use in your taskflows. In a terminal, you can add `AI_API_TOKEN` to the environment like this:

```sh
export AI_API_TOKEN=<your_github_token>

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