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/install/install.go

@@ -30,6 +30,7 @@ func Ensure() error {
 		return err
 	}
 	if samePath(current, target) {
+		_ = syncStartup()
 		return nil
 	}