I am wondering what do you need for writing the program elements to make a total conversion mod. It has been a long while, but if I remember, the Half-Life2 Deathmatch Source included the multiplayer base to get a team play mod working. Under the hood, I think it was the counter-strike general build. I think it was intended to be the template for mods, as well as the multiplayer settings. Basically you just made a copy, put it in the correct mod folder, and edited the (lua?) file. As well as whatever else was needed, as it was a long process. I mainly make level creation, texture, model creation, basic rigging, and some animation.
-edit It turns out the mod creation is done with the Source Sdk tool. Editing it, or creating mod elements past that is what I am trying to find out.