feat(google-antigravity-auth): create auth plugin for Google models

- Implement createGoogleAntigravityAuthPlugin factory function
- Add OAuth method with PKCE for Google authentication
- Create custom fetch interceptor loader for Antigravity API
- Update auth.ts to export Google Antigravity plugin as default
- Update barrel export in antigravity/index.ts
- Add Google Antigravity auth location to AGENTS.md

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AGENTS.md

@@ -34,6 +34,7 @@ oh-my-opencode/
 | Modify LSP behavior | `src/tools/lsp/` | client.ts for connection logic |
 | AST-Grep patterns | `src/tools/ast-grep/` | napi.ts for @ast-grep/napi |
 | Terminal features | `src/features/terminal/` | title.ts |
+| Google Antigravity auth | `src/auth/antigravity/` | OAuth plugin for Google models |
 
 ## CONVENTIONS