fix(namepool): persist custom names to settings/config.json

The gt namepool add command was replacing custom_names instead of
appending because it saved to the runtime state file, but Load()
intentionally ignores CustomNames from that file (expecting config
to come from settings/config.json).

Changes:
- runNamepoolAdd now loads existing settings, appends the new name,
  and saves to settings/config.json (the source of truth)
- runNamepoolSet now preserves existing custom names when changing
  themes (was passing nil which cleared them)
- Added duplicate check to avoid adding same name twice

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/namepool.go

@@ -187,8 +187,17 @@ func runNamepoolSet(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("saving pool: %w", err)
 	}
 
-	// Also save to rig config
-	if err := saveRigNamepoolConfig(rigPath, theme, nil); err != nil {
+	// Load existing settings to preserve custom names when changing theme
+	settingsPath := filepath.Join(rigPath, "settings", "config.json")
+	var existingNames []string
+	if existingSettings, err := config.LoadRigSettings(settingsPath); err == nil {
+		if existingSettings.Namepool != nil {
+			existingNames = existingSettings.Namepool.Names
+		}
+	}
+
+	// Also save to rig config, preserving existing custom names
+	if err := saveRigNamepoolConfig(rigPath, theme, existingNames); err != nil {
 		return fmt.Errorf("saving config: %w", err)
 	}
 
@@ -206,14 +215,44 @@ func runNamepoolAdd(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("not in a rig directory")
 	}
 
-	// Load pool
+	// Load existing rig settings to get current theme and custom names
+	settingsPath := filepath.Join(rigPath, "settings", "config.json")
+	settings, err := config.LoadRigSettings(settingsPath)
+	if err != nil {
+		if os.IsNotExist(err) || strings.Contains(err.Error(), "not found") {
+			settings = config.NewRigSettings()
+		} else {
+			return fmt.Errorf("loading settings: %w", err)
+		}
+	}
+
+	// Initialize namepool config if needed
+	if settings.Namepool == nil {
+		settings.Namepool = config.DefaultNamepoolConfig()
+	}
+
+	// Check if name already exists
+	for _, n := range settings.Namepool.Names {
+		if n == name {
+			fmt.Printf("Name '%s' already in pool\n", name)
+			return nil
+		}
+	}
+
+	// Append new name to existing custom names
+	settings.Namepool.Names = append(settings.Namepool.Names, name)
+
+	// Save to settings/config.json (the source of truth for config)
+	if err := config.SaveRigSettings(settingsPath, settings); err != nil {
+		return fmt.Errorf("saving settings: %w", err)
+	}
+
+	// Also update runtime pool for immediate use
 	pool := polecat.NewNamePool(rigPath, rigName)
 	if err := pool.Load(); err != nil && !os.IsNotExist(err) {
 		return fmt.Errorf("loading pool: %w", err)
 	}
-
 	pool.AddCustomName(name)
-	
 	if err := pool.Save(); err != nil {
 		return fmt.Errorf("saving pool: %w", err)
 	}