fix: support Windows agent command shims

src/agents.ts

@@ -1,4 +1,3 @@
-import { spawn } from "node:child_process";
 import { randomUUID } from "node:crypto";
 import { constants } from "node:fs";
 import { copyFile, cp, mkdir, readFile, readdir, rename, rm, stat, writeFile } from "node:fs/promises";
@@ -6,6 +5,7 @@ import os from "node:os";
 import path from "node:path";
 import { createInterface } from "node:readline/promises";
 import { applyEdits, modify, parse } from "jsonc-parser";
+import spawn from "cross-spawn";
 import type { ClientOptions } from "./config.js";
 import { errorMessage } from "./config.js";
 
@@ -383,8 +383,8 @@ export class SpawnCommandRunner implements CommandRunner {
       const child = spawn(command, args, { shell: false, windowsHide: true, stdio: ["ignore", "pipe", "pipe"] });
       let stdout = "";
       let stderr = "";
-      child.stdout.setEncoding("utf8").on("data", (chunk) => { stdout += String(chunk); });
-      child.stderr.setEncoding("utf8").on("data", (chunk) => { stderr += String(chunk); });
+      child.stdout?.setEncoding("utf8").on("data", (chunk) => { stdout += String(chunk); });
+      child.stderr?.setEncoding("utf8").on("data", (chunk) => { stderr += String(chunk); });
       child.once("error", (error) => reject(new Error(`Could not run ${command}: ${errorMessage(error)}`)));
       child.once("close", (code) => resolve({ code: code ?? 1, stdout, stderr }));
     });