Add authHeader option and fix print mode error handling

- Add 'authHeader' boolean option to models.json provider config
  When true, adds 'Authorization: Bearer <apiKey>' to model headers
  Useful for providers requiring explicit auth headers (fixes #81)

- Fix print mode (-p) silently failing on errors
  Now outputs error message to stderr and exits with code 1
  when assistant message has stopReason of error/aborted

packages/coding-agent/src/model-config.ts

@@ -46,6 +46,7 @@ const ProviderConfigSchema = Type.Object({
 		]),
 	),
 	headers: Type.Optional(Type.Record(Type.String(), Type.String())),
+	authHeader: Type.Optional(Type.Boolean()),
 	models: Type.Array(ModelDefinitionSchema),
 });
 
@@ -177,9 +178,17 @@ function parseModels(config: ModelsConfig): Model<Api>[] {
 			}
 
 			// Merge headers: provider headers are base, model headers override
-			const headers =
+			let headers =
 				providerConfig.headers || modelDef.headers ? { ...providerConfig.headers, ...modelDef.headers } : undefined;
 
+			// If authHeader is true, add Authorization header with resolved API key
+			if (providerConfig.authHeader) {
+				const resolvedKey = resolveApiKey(providerConfig.apiKey);
+				if (resolvedKey) {
+					headers = { ...headers, Authorization: `Bearer ${resolvedKey}` };
+				}
+			}
+
 			models.push({
 				id: modelDef.id,
 				name: modelDef.name,