feat: add support for GitHub Models provider

- Introduced environment variable CLAUDE_CODE_USE_GITHUB to enable GitHub Models.
- Added checks for GITHUB_TOKEN or GH_TOKEN for authentication.
- Updated base URL handling to include GitHub Models default.
- Enhanced provider detection and error handling for GitHub Models.
- Updated relevant functions and components to accommodate the new provider.

src/commands/onboard-github/index.ts

@@ -0,0 +1,11 @@
+import type { Command } from '../../commands.js'
+
+const onboardGithub: Command = {
+  name: 'onboard-github',
+  description:
+    'Interactive setup for GitHub Models: device login or PAT, saved to secure storage',
+  type: 'local-jsx',
+  load: () => import('./onboard-github.js'),
+}
+
+export default onboardGithub