I just finished a very similar project! I was very interested in @ogiewon’s Arduino project, but unfortunately the ThingShield is no longer available and I couldn’t get my hands on one. Instead, I went with a NodeMCU ESP8266 board with built in wifi. The ST_Anything project was a great inspiration, but with the NodeMCU it communicates directly to ST API over your wifi connection, so the flow of information is a bit different.
Overall I’m very happy with the setup and low cost of the project. Works like a charm.
I wrote up some details of the project here and there is code linked on Github: Connect wired alarm system sensors to SmartThings with a NodeMCU ESP8266 [deprecated]