fix: exit non-zero and print help to stderr for bare auth/root invocations

cmd/auth.go

@@ -12,8 +12,14 @@ import (
 )
 
 var authCmd = &cobra.Command{
-	Use:   "auth",
-	Short: "Authentication commands",
+	Use:          "auth",
+	Short:        "Authentication commands",
+	SilenceUsage: true,
+	RunE: func(cmd *cobra.Command, args []string) error {
+		cmd.SetOut(cmd.ErrOrStderr())
+		_ = cmd.Help()
+		return fmt.Errorf("subcommand required")
+	},
 }
 
 var loginCmd = &cobra.Command{

cmd/root.go

@@ -1,21 +1,28 @@
 package cmd
 
 import (
+	"fmt"
 	"os"
 
 	"github.com/spf13/cobra"
 )
 
 var rootCmd = &cobra.Command{
-	Use:   "withings-export",
-	Short: "CLI to export health data from Withings",
+	Use:          "withings-export",
+	Short:        "CLI to export health data from Withings",
+	SilenceUsage: true,
 	Long: `withings-export reads your personal Withings health data — activity,
 sleep, workouts, body measurements, intraday samples — and prints it on
 stdout. Default output is narrow, fitdown-style markdown; pass
 --format json or --format csv for structured output.
 
 LLM agents: run 'withings-export prime' for a one-screen orientation
 (I/O contract, subcommands, date flags, jq recipes).`,
+	RunE: func(cmd *cobra.Command, args []string) error {
+		cmd.SetOut(cmd.ErrOrStderr())
+		_ = cmd.Help()
+		return fmt.Errorf("subcommand required")
+	},
 }
 
 func Execute() {