Add waitForExit() method to Process objects

[ghostwriternr]

Problem

When using startProcess() for tasks that exit on their own, there's no clean way to wait for completion. Users must either:

1. Use waitForLog() with a known output pattern
2. Manually wrap onExit callback in a Promise

// Current workaround
let resolveExit: (code: number | null) => void;
const exitPromise = new Promise<number | null>((r) => resolveExit = r);
const process = await sandbox.startProcess('npm run build', { onExit: resolveExit });
await exitPromise;

Proposal

Add waitForExit(timeout?) to Process objects, complementing existing waitForLog() and waitForPort():

const process = await sandbox.startProcess('npm run build');
const exitCode = await process.waitForExit(60000);

Use Cases

• Run multiple processes concurrently, wait for specific ones to finish
• Start a process, do other work, then wait for completion
• Start a process that might run indefinitely, but check if it crashed early