Add virtual model support for multi-key expansion

Virtual models generated from multi-key expansion are now marked and
filtered during config persistence. Virtual models display with a badge
in the UI and cannot be set as default.

pkg/config/config.go

@@ -857,6 +857,10 @@ type ModelConfig struct {
 	secModelName string
 	apiKeys      []string
 	secDirty     bool
+
+	// isVirtual marks this model as a virtual model generated from multi-key expansion.
+	// Virtual models should not be persisted to config files.
+	isVirtual bool
 }
 
 // APIKey returns the first API key from apiKeys
@@ -867,6 +871,11 @@ func (c *ModelConfig) APIKey() string {
 	return ""
 }
 
+// IsVirtual returns true if this model was generated from multi-key expansion.
+func (c *ModelConfig) IsVirtual() bool {
+	return c.isVirtual
+}
+
 // Validate checks if the ModelConfig has all required fields.
 func (c *ModelConfig) Validate() error {
 	if c.ModelName == "" {
@@ -1800,7 +1809,20 @@ func SaveConfig(path string, cfg *Config) error {
 		return err
 	}
 
+	// Filter out virtual models before serializing to config file
+	nonVirtualModels := make([]*ModelConfig, 0, len(cfg.ModelList))
+	for _, m := range cfg.ModelList {
+		if !m.isVirtual {
+			nonVirtualModels = append(nonVirtualModels, m)
+		}
+	}
+	// Temporarily replace ModelList with filtered version for serialization
+	originalModelList := cfg.ModelList
+	cfg.ModelList = nonVirtualModels
+
 	data, err := json.MarshalIndent(cfg, "", "  ")
+	// Restore original ModelList after serialization
+	cfg.ModelList = originalModelList
 	if err != nil {
 		return err
 	}
@@ -2018,6 +2040,7 @@ func expandMultiKeyModels(models []*ModelConfig) []*ModelConfig {
 				RequestTimeout: m.RequestTimeout,
 				ThinkingLevel:  m.ThinkingLevel,
 				ExtraBody:      m.ExtraBody,
+				isVirtual:      true,
 			}
 			expanded = append(expanded, additionalEntry)
 			fallbackNames = append(fallbackNames, expandedName)