Document process execution helpers and add example

[JBloss1517]

Summary

• document when to reach for run_sync_in_process and run_async_in_process and outline their trade-offs
• add a runnable process-pool example showcasing shared executors and async dependencies
• mark process entry points as no-cover since they execute inside worker processes

Testing

• uv run poe check-all

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https://chatgpt.com/codex/tasks/task_e_6902324d538c832fbb7ba62bb64c1315