feat: interactive setup wizard for first-run and netssh configure

- Auto-detects missing config (netbox.url empty) and launches wizard
- `netssh configure` re-runs the wizard anytime to change settings
- 3-page huh form: NetBox connection, SSH defaults, resolver & cache
- Saves to ~/.config/netssh.yaml (permissions 0600)

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

internal/config/config.go

@@ -35,6 +35,15 @@ type SSHConfig struct {
 	DefaultUser string `mapstructure:"default_user"`
 }
 
+// Path returns the canonical config file path.
+func Path() string {
+	configDir, err := os.UserConfigDir()
+	if err != nil {
+		configDir = filepath.Join(os.Getenv("HOME"), ".config")
+	}
+	return filepath.Join(configDir, "netssh.yaml")
+}
+
 func Load() (*Config, error) {
 	v := viper.New()