Add background-workers agent example with Node.js Worker Threads

[cerashdan]

This commit adds a new sample demonstrating how to use Node.js Worker Threads to handle CPU-blocking operations without freezing the agent's main thread. Due to the single-thread nature of Node JS, any blocking operation such as file operations and loop will block the AI agent from receiving new incoming requests.

New Features:

• Background worker agent using worker_threads for CPU-intensive tasks
• Fire-and-forget pattern: agent returns immediately without waiting for workers
• get_task_status tool using app.getAsyncTaskInfo() for monitoring active tasks
• Non-blocking architecture allowing concurrent request handling
• Comprehensive documentation on blocking vs non-blocking operations

Changes:

• primitives/runtime/background-workers/: New sample with agent, worker, and docs
• primitives/runtime/async-agent/: Added get_task_status tool for consistency
• primitives/runtime/README.md: Added link to new background-workers sample

The example demonstrates:

• Worker threads for blocking operations (loops, dataset processing, calculations)

• Main thread stays responsive while workers process in background

• Multiple concurrent workers can run simultaneously

• Health status tracking (Healthy/HealthyBusy)

• Message passing between main thread and worker threads

• New example

• Bug fix

• Documentation update

• Configuration/tooling update

• Other (describe below)

Related Issues

Closes #16

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the existing style patterns
• I have run npm run validate locally and it passes
• I have added/updated documentation as needed
• I have tested my changes locally