SDK spawn reports misleading ENOENT when cwd does not exist

Summary

When query() is called with a cwd that does not exist on the filesystem, Node.js child_process.spawn() throws:

Error: Failed to spawn Claude Code process: spawn /opt/homebrew/bin/claude ENOENT

This error strongly implies the executable is missing, but the actual cause is the working directory does not exist. This is extremely misleading and leads users down the wrong debugging path (reinstalling Claude Code, changing executable paths, rolling back SDK versions, etc.).

Steps to Reproduce

import { query } from "@anthropic-ai/claude-agent-sdk";

const stream = query({
  prompt: "hello",
  options: {
    cwd: "/nonexistent/directory",  // <-- this is the real problem
    pathToClaudeCodeExecutable: "/opt/homebrew/bin/claude",  // <-- error blames this
  }
});

Expected Behavior

The SDK should validate that cwd exists before calling spawn(), and throw a clear error like:

Error: Working directory does not exist: /nonexistent/directory

Actual Behavior

Error: Failed to spawn Claude Code process: spawn /opt/homebrew/bin/claude ENOENT

Environment

• SDK version: 0.2.62
• Node.js: v25.6.1
• OS: macOS (Darwin 25.3.0, arm64)

Root Cause

This is a known Node.js behavior — spawn() reports ENOENT with the executable path even when the real issue is an invalid cwd. The SDK already validates the executable with existsSync() before spawning, but does not validate cwd.

Suggested Fix

Add a cwd existence check before the spawn call, similar to the existing executable check:

if (cwd && !fs.existsSync(cwd)) {
  throw new Error(`Working directory does not exist: ${cwd}`);
}