feat: add agent cost reporting

hub/src/agent/cost.ts

@@ -0,0 +1,23 @@
+export function formatRunCostLine(costUsd: number | undefined): string {
+  return costUsd === undefined
+    ? "本次成本: 未记录"
+    : `本次成本: ${formatUsd(costUsd)}`;
+}
+
+export function formatInteger(value: number): string {
+  return new Intl.NumberFormat("en-US").format(value);
+}
+
+export function formatUsd(value: number): string {
+  const digits = value > 0 && value < 0.01 ? 4 : 2;
+  return `$${value.toFixed(digits)}`;
+}
+
+export function decimalToNumberOrNull(value: unknown): number | null {
+  if (value === null || value === undefined) return null;
+  const numberValue = typeof value === "number" ? value : Number(value);
+  if (!Number.isFinite(numberValue)) {
+    throw new Error("invalid cost value");
+  }
+  return numberValue;
+}