Remove sunset GitHub Models integration

[Copilot]

GitHub Models inference has been sunset. Remove the obsolete integration and route supported model requests exclusively through GitHub Copilot.

Changes

• Routing

  • Removed publisher-qualified model routing to GitHub Models.
  • Simplified chat and Gemini upstream selection to Copilot only.

• Configuration

  • Removed github_models settings and environment variables.
  • Bumped the schema to v6 so legacy configuration is rewritten without retired fields.

• Authentication and discovery

  • Removed dedicated Models token validation and setup prompts.
  • Removed GitHub Models catalog merging from /v1/models.

• Documentation

  • Removed GitHub Models setup, authentication, routing, and configuration guidance.
  • Added the removal to the changelog.