feat: persist token_version in config, auto-detect on load

- NetBoxConfig.TokenVersion saved to netssh.yaml by the wizard
- config.Load() auto-detects the version from the token prefix if the
  field is missing (backwards-compatible with existing configs)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/spf13/viper"
 )
@@ -16,8 +17,9 @@ type Config struct {
 }
 
 type NetBoxConfig struct {
-	URL   string `mapstructure:"url"`
-	Token string `mapstructure:"token"`
+	URL          string `mapstructure:"url"`
+	Token        string `mapstructure:"token"`
+	TokenVersion int    `mapstructure:"token_version"`
 }
 
 type ResolverConfig struct {
@@ -73,6 +75,14 @@ func Load() (*Config, error) {
 		return nil, fmt.Errorf("parsing config: %w", err)
 	}
 
+	if cfg.NetBox.TokenVersion == 0 && cfg.NetBox.Token != "" {
+		if strings.HasPrefix(cfg.NetBox.Token, "nbt_") {
+			cfg.NetBox.TokenVersion = 2
+		} else {
+			cfg.NetBox.TokenVersion = 1
+		}
+	}
+
 	if cfg.Cache.Path == "" {
 		cacheDir, err := os.UserCacheDir()
 		if err != nil {