Background Tasks: The One Actor in the Codebase and the SIGTERM Bug That Only Broke on Linux

A new BackgroundManager actor was introduced to handle slow commands in the background, reducing the bottleneck caused by sequential execution. This allows the agent loop to continue running while slow commands are executed in parallel. The BackgroundManager uses Swift's actor keyword to prevent data races. Engineers should use the BackgroundManager to improve performance and efficiency in the codebase.