fix(interactive-subagents): harden parallel lifecycle delivery

extensions/interactive-subagents/pi-extension/subagents/subagent-done.ts

@@ -15,7 +15,7 @@
 import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
 import { Box, Text } from "@earendil-works/pi-tui";
 import { Type } from "typebox";
-import { writeFileSync } from "node:fs";
+import { renameSync, writeFileSync } from "node:fs";
 import { createSubagentActivityRecorder } from "./activity.ts";
 
 export function shouldMarkUserTookOver(agentStarted: boolean): boolean {
@@ -373,7 +373,10 @@ export default function (pi: ExtensionAPI) {
         agent: process.env.PI_SUBAGENT_AGENT ?? "",
         question: params.question,
       };
-      writeFileSync(`${sessionFile}.ask`, JSON.stringify(askData));
+      const askFile = `${sessionFile}.ask`;
+      const tempAskFile = `${askFile}.tmp-${process.pid}-${Math.random().toString(16).slice(2, 8)}`;
+      writeFileSync(tempAskFile, JSON.stringify(askData));
+      renameSync(tempAskFile, askFile);
 
       return {
         content: [