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/daemon/daemon.go

@@ -127,7 +127,7 @@ func (d *Daemon) Run() error {
 
 	// Handle signals
 	sigChan := make(chan os.Signal, 1)
-	signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM, syscall.SIGUSR1)
+	signal.Notify(sigChan, daemonSignals()...)
 
 	// Fixed recovery-focused heartbeat (no activity-based backoff)
 	// Normal wake is handled by feed subscription (bd activity --follow)
@@ -162,9 +162,9 @@ func (d *Daemon) Run() error {
 			return d.shutdown(state)
 
 		case sig := <-sigChan:
-			if sig == syscall.SIGUSR1 {
-				// SIGUSR1: immediate lifecycle processing (from gt handoff)
-				d.logger.Println("Received SIGUSR1, processing lifecycle requests immediately")
+			if isLifecycleSignal(sig) {
+				// Lifecycle signal: immediate lifecycle processing (from gt handoff)
+				d.logger.Println("Received lifecycle signal, processing lifecycle requests immediately")
 				d.processLifecycleRequests()
 			} else {
 				d.logger.Printf("Received signal %v, shutting down", sig)