fix: Windows build support with platform-specific process/signal handling

Separate platform-dependent code into build-tagged files:
- process_unix.go / process_windows.go: isProcessRunning() implementation
- signals_unix.go / signals_windows.go: daemon signal handling (Windows lacks SIGUSR1)

Windows implementation uses windows.OpenProcess with PROCESS_QUERY_LIMITED_INFORMATION
and checks exit code against STILL_ACTIVE (259).

Original-PR: #447
Co-Authored-By: Johann Dirry <johann.dirry@microsea.at>

internal/cmd/down.go

@@ -6,7 +6,6 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"syscall"
 	"time"
 
 	"github.com/gofrs/flock"
@@ -451,19 +450,3 @@ func verifyShutdown(t *tmux.Tmux, townRoot string) []string {
 
 	return respawned
 }
-
-// isProcessRunning checks if a process with the given PID exists.
-func isProcessRunning(pid int) bool {
-	if pid <= 0 {
-		return false // Invalid PID
-	}
-	err := syscall.Kill(pid, 0)
-	if err == nil {
-		return true
-	}
-	// EPERM means process exists but we don't have permission to signal it
-	if err == syscall.EPERM {
-		return true
-	}
-	return false
-}