fix(cli): ensure child process is killed on exit

[pi0]

resolves #111 also fixes graceful stop to allow .waitUntil() working properly

import { spawn } from "node:child_process";

const child = spawn("./bin/srvx.mjs", ["examples/hello-world"], {
  stdio: "inherit",
});

console.log("> Spawned child pid:", child.pid);
console.log("> Waiting 500ms");

await new Promise((r) => setTimeout(r, 500));

console.log('> Killing with "SIGINT"', child.kill());

[codecov]

Codecov Report

❌ Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/cli.ts	0.00%	30 Missing ⚠️

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