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/coding-agent/test/suite/regressions/6596-taskkill-enoent.test.ts

@@ -0,0 +1,52 @@
+import type { ChildProcess } from "node:child_process";
+import { EventEmitter } from "node:events";
+import { join } from "node:path";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+const { spawnMock } = vi.hoisted(() => ({ spawnMock: vi.fn() }));
+
+vi.mock("child_process", async (importOriginal) => {
+	const actual = await importOriginal<typeof import("child_process")>();
+	return { ...actual, spawn: spawnMock };
+});
+
+import { killProcessTree } from "../../../src/utils/shell.ts";
+
+function withWindowsPlatform(test: () => void): void {
+	const platformDescriptor = Object.getOwnPropertyDescriptor(process, "platform");
+	try {
+		Object.defineProperty(process, "platform", { configurable: true, value: "win32" });
+		test();
+	} finally {
+		if (platformDescriptor) Object.defineProperty(process, "platform", platformDescriptor);
+	}
+}
+
+afterEach(() => {
+	spawnMock.mockReset();
+});
+
+describe("issue #6596 taskkill spawn failures", () => {
+	it("uses System32 taskkill and consumes its asynchronous spawn error", () => {
+		const child = new EventEmitter() as ChildProcess;
+		const previousSystemRoot = process.env.SystemRoot;
+		process.env.SystemRoot = "C:\\CustomWindows";
+		spawnMock.mockReturnValue(child);
+
+		try {
+			withWindowsPlatform(() => {
+				killProcessTree(1234);
+			});
+		} finally {
+			if (previousSystemRoot === undefined) delete process.env.SystemRoot;
+			else process.env.SystemRoot = previousSystemRoot;
+		}
+
+		expect(spawnMock).toHaveBeenCalledWith(
+			join("C:\\CustomWindows", "System32", "taskkill.exe"),
+			["/F", "/T", "/PID", "1234"],
+			{ detached: true, stdio: "ignore", windowsHide: true },
+		);
+		expect(() => child.emit("error", new Error("spawn taskkill ENOENT"))).not.toThrow();
+	});
+});