fix(startup): make watchdog task keep agent alive

- Use Task Scheduler XML instead of /TR
- Run -ensure agent in-process, not child
- Add console close and panic protection
- Guard nil event channels after stop
- Add non-Windows stubs for startup/mutex

lib/instance/detach.go

@@ -1,7 +1,6 @@
 package instance
 
 import (
-	"fmt"
 	"os"
 
 	"tea.chunkbyte.com/kato/go-worm/lib/config"
@@ -9,7 +8,7 @@ import (
 
 func Detach() error {
 	if AlreadyRunning() {
-		return fmt.Errorf("agent already running")
+		return nil
 	}
 	exe, err := config.InstalledExe()
 	if err != nil {