Split platform-specific functions into separate files:
- cmd_unix.go: Unix implementation using Setpgid and signal 0
- cmd_windows.go: Windows implementation using CREATE_NEW_PROCESS_GROUP
and OpenProcess for PID checking
Fixes Windows cross-compilation error where Setpgid field doesn't exist.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>