fix(config): remove hardcoded default model, use first from model_list

Previously, DefaultConfig() had Model: "glm-4.7" as a hardcoded default.
When users removed model_name from their config, this default was used,
causing confusion because:
1. The user had no idea where "glm-4.7" came from
2. Config changes appeared to not be picked up
3. The system tried to use an unconfigured model

Changes:
1. Remove hardcoded "glm-4.7" default from DefaultConfig()
2. When model is empty, use the first model_name from model_list
3. Provide clear error message if no models are available

This also fixes the issue where config changes weren't being picked up
after restart - the user's missing model_name was being replaced with
the hardcoded default.

Users should now explicitly specify model_name in their config.

pkg/config/defaults.go

@@ -13,7 +13,7 @@ func DefaultConfig() *Config {
 				Workspace:           "~/.picoclaw/workspace",
 				RestrictToWorkspace: true,
 				Provider:            "",
-				Model:               "glm-4.7",
+				Model:               "", // Require explicit model specification
 				MaxTokens:           8192,
 				Temperature:         nil, // nil means use provider default
 				MaxToolIterations:   20,

pkg/providers/legacy_provider.go

@@ -28,6 +28,15 @@ func CreateProvider(cfg *config.Config) (LLMProvider, string, error) {
 		return nil, "", fmt.Errorf("no providers configured. Please add entries to model_list in your config")
 	}
 
+	// If no model is specified, use the first model from model_list
+	if model == "" {
+		if len(cfg.ModelList) > 0 {
+			model = cfg.ModelList[0].ModelName
+		} else {
+			return nil, "", fmt.Errorf("no model specified and no models in model_list. Please set model_name in agents.defaults or add models to model_list")
+		}
+	}
+
 	// Get model config from model_list
 	modelCfg, err := cfg.GetModelConfig(model)
 	if err != nil {