refactor(agents): dedupe lifecycle send assertions and stable payload stringify

src/agents/stable-stringify.ts

@@ -0,0 +1,12 @@
+export function stableStringify(value: unknown): string {
+  if (value === null || typeof value !== "object") {
+    return JSON.stringify(value) ?? "null";
+  }
+  if (Array.isArray(value)) {
+    return `[${value.map((entry) => stableStringify(entry)).join(",")}]`;
+  }
+  const record = value as Record<string, unknown>;
+  const keys = Object.keys(record).toSorted();
+  const entries = keys.map((key) => `${JSON.stringify(key)}:${stableStringify(record[key])}`);
+  return `{${entries.join(",")}}`;
+}