Fix Windows agent self-update restart failure

syscall.Exec is not supported on Windows, causing self-update to fail
with "failed to restart: not supported by windows".

Split restart logic into platform-specific files:
- restart_unix.go: Uses syscall.Exec for in-place process replacement
- restart_windows.go: Uses os.Exit(0) to let Windows SCM restart service

Related to #735

internal/agentupdate/restart_unix.go

@@ -0,0 +1,22 @@
+//go:build !windows
+
+package agentupdate
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+// restartProcess replaces the current process with a new instance.
+// On Unix-like systems, this uses syscall.Exec for an in-place restart.
+func restartProcess(execPath string) error {
+	args := os.Args
+	env := os.Environ()
+
+	if err := syscall.Exec(execPath, args, env); err != nil {
+		return fmt.Errorf("failed to restart: %w", err)
+	}
+
+	return nil
+}

internal/agentupdate/restart_windows.go

@@ -0,0 +1,22 @@
+//go:build windows
+
+package agentupdate
+
+import (
+	"os"
+)
+
+// restartProcess triggers a restart on Windows.
+// For Windows services, we exit cleanly and rely on the Service Control Manager
+// to restart the service (services are typically configured with automatic recovery).
+// For non-service processes, os.Exit(0) is still appropriate as the process will
+// restart with the new binary on next invocation.
+func restartProcess(execPath string) error {
+	// Exit with code 0 to signal clean shutdown.
+	// Windows Service Control Manager will restart the service if configured
+	// for automatic recovery (which is the default for our PowerShell installer).
+	os.Exit(0)
+
+	// This line is never reached, but satisfies the compiler
+	return nil
+}

internal/agentupdate/update.go

@@ -18,7 +18,6 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
-	"syscall"
 	"time"
 
 	"github.com/rcourtman/pulse-go-rewrite/internal/utils"
@@ -430,15 +429,8 @@ func (u *Updater) performUpdate(ctx context.Context) error {
 		}
 	}
 
-	// Restart with same arguments
-	args := os.Args
-	env := os.Environ()
-
-	if err := syscall.Exec(execPath, args, env); err != nil {
-		return fmt.Errorf("failed to restart: %w", err)
-	}
-
-	return nil
+	// Restart the process using platform-specific implementation
+	return restartProcess(execPath)
 }
 
 // determineArch returns the architecture string for download URLs (e.g., "linux-amd64", "darwin-arm64").