Since no one has mentioned it in this thread, I'd like to mention that the zombie code does also not contain the references to animations for breaking in half, nor detatchment of headcrab. (yet later, this function that doesn't have an animation is called (both detachment of headcrabs and legs)).
If I'm not mistaken, Half-Life used the Quake engine, which has been released to the public. The SDK can be modified for DLLs, and the Quake source for the exes.
I've heard from multiple accounts that the AI for the grunt is identical in both versions, that there's folders that shouldn't exist if it was the real one they had, and I'm thinking that it is just the Half-Life code editted by people with a lot of time on their hands.