fix: initialize tools slice to empty rather than nil in registry

internal/registry/registry.go

@@ -27,7 +27,10 @@ type Registry struct {
 }
 
 func New() *Registry {
-	return &Registry{skills: make(map[string]Skill)}
+	return &Registry{
+		skills: make(map[string]Skill),
+		tools:  make([]ToolDef, 0),
+	}
 }
 
 func (r *Registry) Register(s Skill) {