feat: workers() defaults to number of available CPUs

Co-Authored-By: Claude Opus 4.6 (1M context) <[e-mail]>

src/worker-pool.ts

···
 import { Worker } from 'node:worker_threads';
+import { availableParallelism } from 'node:os';
 import { setupWorker, execute } from './execute.ts';
 import type { Task } from './task.ts';
 import type { Runner } from './runner.ts';
···
 
 /**
  * Creates a pool of worker threads that dispatch tasks with round-robin scheduling.
- * @param size - Number of worker threads in the pool.
+ * @param size - Number of worker threads in the pool. Defaults to the number of available CPUs.
  * @returns A disposable {@link Runner} for dispatching tasks.
  */
-export function workers(size: number): Runner {
+export function workers(size: number = availableParallelism()): Runner {
   const pool: Worker[] = [];
   for (let i = 0; i < size; i++) {
     const worker = new Worker(workerEntryUrl);