Add Windows support

[ajw1980]

Add Windows support

Closes #102

Adds cross-compilation and runtime support for Windows. Tested running as a Windows service with a real crontab firing on schedule.

Approach

Build tags are used only where a symbol literally doesn't exist on Windows and would cause a compile error regardless of code path (Setpgid, SIGUSR2, ForkExec, Wait4, SIGCHLD). Everything else uses runtime.GOOS to keep the number of new files minimal.

Three new files:

• cron/unix.go / cron/windows.go — platform split for sysProcAttr() since Setpgid won't compile on Windows even in dead code
• windows.go — all main package Windows stubs in one place: forkExec no-op, synthetic reloadSig sentinel, and a Windows-appropriate signalList

Four modified files:

• reaper.go — //go:build !windows tag; reloadSignal and signalList moved here for the Unix side
• cron/cron.go — sysProcAttr() call; runtime.GOOS to select -c vs -Command shell flag
• crontab/crontab.go — runtime.GOOS to default to powershell.exe on Windows instead of /bin/sh
• main.go — syscall.SIGUSR2 replaced with reloadSignal throughout; syscall import removed

Platform notes

Default shell: powershell.exe with -Command. Override with SHELL= in your crontab as usual.

Crontab reload: SIGUSR2 can't be delivered to a Windows process externally so that path is a no-op on Windows. The -inotify flag works correctly via ReadDirectoryChangesW and is the recommended reload mechanism on Windows.

Process reaping: forkExec() is stubbed with logrus.Fatal. The reaper is only entered when os.Getpid() == 1, which is never true on Windows, so it's dead code in practice.