chore(vendor): import pi 0.85.1

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: d981de1229ef899957bbe968bc8dcda02a21f477

third_party/pi/packages/agent/test/harness/session-test-utils.ts

@@ -0,0 +1,20 @@
+import { existsSync, mkdirSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach } from "vitest";
+
+const tempDirs: string[] = [];
+
+export function createTempDir(): string {
+	const dir = join(tmpdir(), `pi-agent-session-${Date.now()}-${Math.random().toString(36).slice(2)}`);
+	mkdirSync(dir, { recursive: true });
+	tempDirs.push(dir);
+	return dir;
+}
+
+afterEach(() => {
+	while (tempDirs.length > 0) {
+		const dir = tempDirs.pop()!;
+		if (existsSync(dir)) rmSync(dir, { recursive: true, force: true });
+	}
+});