fix: improve migration logic and reduce code duplication

- Preserve user's configured model during config migration (issue #5)
- Simplify ExtractProtocol using strings.Cut
- Extract NormalizeToolCall to shared utility, removing ~70 lines of duplicate code
- Clean up unused fields in providerMigrationConfig struct

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

pkg/providers/factory_provider.go

@@ -45,13 +45,11 @@ func createCodexAuthProvider() (LLMProvider, error) {
 //   - "gpt-4o" -> ("openai", "gpt-4o")  // default protocol
 func ExtractProtocol(model string) (protocol, modelID string) {
 	model = strings.TrimSpace(model)
-	for i := 0; i < len(model); i++ {
-		if model[i] == '/' {
-			return model[:i], model[i+1:]
-		}
+	protocol, modelID, found := strings.Cut(model, "/")
+	if !found {
+		return "openai", model
 	}
-	// No prefix found, default to openai
-	return "openai", model
+	return protocol, modelID
 }
 
 // CreateProviderFromConfig creates a provider based on the ModelConfig.

pkg/providers/toolcall_utils.go

@@ -0,0 +1,54 @@
+// PicoClaw - Ultra-lightweight personal AI agent
+// License: MIT
+//
+// Copyright (c) 2026 PicoClaw contributors
+
+package providers
+
+import "encoding/json"
+
+// NormalizeToolCall normalizes a ToolCall to ensure all fields are properly populated.
+// It handles cases where Name/Arguments might be in different locations (top-level vs Function)
+// and ensures both are populated consistently.
+func NormalizeToolCall(tc ToolCall) ToolCall {
+	normalized := tc
+
+	// Ensure Name is populated from Function if not set
+	if normalized.Name == "" && normalized.Function != nil {
+		normalized.Name = normalized.Function.Name
+	}
+
+	// Ensure Arguments is not nil
+	if normalized.Arguments == nil {
+		normalized.Arguments = map[string]interface{}{}
+	}
+
+	// Parse Arguments from Function.Arguments if not already set
+	if len(normalized.Arguments) == 0 && normalized.Function != nil && normalized.Function.Arguments != "" {
+		var parsed map[string]interface{}
+		if err := json.Unmarshal([]byte(normalized.Function.Arguments), &parsed); err == nil && parsed != nil {
+			normalized.Arguments = parsed
+		}
+	}
+
+	// Ensure Function is populated with consistent values
+	argsJSON, _ := json.Marshal(normalized.Arguments)
+	if normalized.Function == nil {
+		normalized.Function = &FunctionCall{
+			Name:      normalized.Name,
+			Arguments: string(argsJSON),
+		}
+	} else {
+		if normalized.Function.Name == "" {
+			normalized.Function.Name = normalized.Name
+		}
+		if normalized.Name == "" {
+			normalized.Name = normalized.Function.Name
+		}
+		if normalized.Function.Arguments == "" {
+			normalized.Function.Arguments = string(argsJSON)
+		}
+	}
+
+	return normalized
+}