Shell Process Tracking Decoupled from Background Tools

Description

This PR implements a dedicated ShellTracker class to handle shell process tracking, decoupling it from the general backgroundTools implementation. This change improves modularity and separation of concerns.

Changes

• Created a new ShellTracker class with methods for tracking shell processes
• Implemented a new listShells tool to list shell processes
• Updated shellStart and shellMessage to use the new ShellTracker instead of backgroundTools
• Modified backgroundTools to remove shell-specific functionality and use ShellTracker for cleanup
• Updated listBackgroundTools to exclude shell processes (now handled by listShells)
• Added tests for the new ShellTracker class and listShells tool
• Updated existing tests to work with the new structure

Testing

All tests are passing. The implementation maintains backward compatibility while providing a cleaner separation of concerns.

Additional Notes

This change is part of a larger effort to make the codebase more modular and easier to maintain.