Extract platform-specific syscall code into proc_unix.go and proc_windows.go
with appropriate build tags. This fixes TestCrossPlatformBuild which failed
because syscall.SysProcAttr.Setpgid is Unix-only.
Changes:
- setSysProcAttr(): Sets process group on Unix, no-op on Windows
- isProcessAlive(): Uses Signal(0) on Unix, nil signal on Windows
- sendTermSignal(): SIGTERM on Unix, Kill() on Windows
- sendKillSignal(): SIGKILL on Unix, Kill() on Windows
Fixes gt-3078ed
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>