docs: fix gemini-cli authentication documentation (#923)

Remove erroneous 'gemini auth login' command references and replace with correct 'gemini' command authentication flow. Update documentation to reflect proper OAuth setup process via the gemini CLI interactive interface.

src/ai-providers/gemini-cli.js

@@ -67,8 +67,7 @@ export class GeminiCliProvider extends BaseAIProvider {
 					apiKey: params.apiKey
 				};
 			} else {
-				// Expected case: Use gemini CLI authentication
-				// Requires: gemini auth login (pre-configured)
+				// Expected case: Use gemini CLI authentication via OAuth
 				authOptions = {
 					authType: 'oauth-personal'
 				};