fix: add background PID tracking support for Windows

[akramcodez]

Description

Background PID tracking was not supported on Windows - the backgroundPids field was always empty because the Unix pgrep logic was skipped on Windows.

Solution

• Added getWindowsChildPids() using PowerShell's Get-CimInstance
• Uses modern WMI query instead of deprecated WMIC
• Graceful error handling with fallback to empty array

Changes

• shell.ts: Added Windows PID detection using PowerShell

Testing

Verified on mock Windows environment - function executes and handles errors gracefully.

fixes #12