Replace killProcessTree references to shell-utils with process/kill-tree (#55213)

* Replace killProcessTree references to shell-utils with process/kill-tree

* Address grace timeout comment

* Align with existing process kill behavior

* bash: fail stop without pid

* bash: lazy-load kill tree on stop

---------

Co-authored-by: Jacob Tomlinson <jtomlinson@nvidia.com>

src/agents/bash-tools.exec.background-abort.test.ts

@@ -1,11 +1,11 @@
 import { afterEach, expect, test } from "vitest";
+import { killProcessTree } from "../process/kill-tree.js";
 import {
   getFinishedSession,
   getSession,
   resetProcessRegistryForTests,
 } from "./bash-process-registry.js";
 import { createExecTool } from "./bash-tools.exec.js";
-import { killProcessTree } from "./shell-utils.js";
 
 const BACKGROUND_HOLD_CMD = 'node -e "setTimeout(() => {}, 5000)"';
 const ABORT_SETTLE_MS = process.platform === "win32" ? 200 : 25;

src/agents/shell-utils.ts

@@ -1,4 +1,3 @@
-import { spawn } from "node:child_process";
 import fs from "node:fs";
 import path from "node:path";
 
@@ -166,27 +165,3 @@ export function sanitizeBinaryOutput(text: string): string {
   }
   return chunks.join("");
 }
-
-export function killProcessTree(pid: number): void {
-  if (process.platform === "win32") {
-    try {
-      spawn("taskkill", ["/F", "/T", "/PID", String(pid)], {
-        stdio: "ignore",
-        detached: true,
-      });
-    } catch {
-      // ignore errors if taskkill fails
-    }
-    return;
-  }
-
-  try {
-    process.kill(-pid, "SIGKILL");
-  } catch {
-    try {
-      process.kill(pid, "SIGKILL");
-    } catch {
-      // process already dead
-    }
-  }
-}