Remove the GitHub Copilot provider (forked SDK + plugin) #187

Decision

Remove GitHub Copilot as a built-in provider: packages/cli/src/provider/sdk/copilot/ (~4.4k lines, a forked OpenAI-compatible SDK marked "TODO: kill this code"), src/plugin/copilot.ts (~330 lines), and the Copilot handling in provider/provider.ts (BUNDLED_PROVIDERS @ai-sdk/github-copilot, the github-copilot(-enterprise) custom loaders and the enterprise clone), auth and models commands, and transform.ts. Other OpenAI-compatible providers are unaffected. Stored Copilot auth entries are ignored.

Rules

• Delete the code, its tests and any dependencies only it uses. The net diff should be strongly negative. No default-off switch and no opt-in to bring it back.
• Compatibility shims only where existing configs or stored data would otherwise break:
  • config keys still load as deprecated no-ops, with one warning;
  • old sessions still load;
  • removed commands fail with a clear "was removed" error.
• Tracked under the headless optimisation epic Epic: Optimise headless run execution #153.