agent: default sing-box stats API port 10086 -> 10089

Avoids the v2ray_api listen colliding with L-UI's fixed 10086 on
dual-homed nodes. NG_SINGBOX_API_PORT env still overrides. Aligns the
stats-query --server default and the doctor script fallback to the
same value. Agent version stays 0.6.8.

agent/stats-query.go

@@ -16,7 +16,7 @@ import (
 )
 
 func main() {
-	server := flag.String("server", "127.0.0.1:10086", "sing-box statistics API address")
+	server := flag.String("server", "127.0.0.1:10089", "sing-box statistics API address")
 	flag.Parse()
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()