Add modify_file tool

Removed file_insert_lines, file_replace_lines

pkg/agents/toolbox/util/util.go

@@ -65,3 +65,14 @@ func IsPathWithinCWD(path string) (bool, string) {
 	}
 	return true, ""
 }
+
+// AddLineNumbers takes a string of content and returns a new string with line
+// numbers prefixed
+func AddLineNumbers(content string) string {
+	lines := strings.Split(strings.TrimSuffix(content, "\n"), "\n")
+	result := strings.Builder{}
+	for i, line := range lines {
+		result.WriteString(fmt.Sprintf("%d\t%s\n", i+1, line))
+	}
+	return result.String()
+}