You'll have to get rid of that snake asap. Is there a snake park or something similar near you? If so, phone them and ask them if they can send someone with experience handling snakes over to remove it for you. And make sure your brooder is secure so it can't get in there. Good luck!
If its a black rat snake, they're not very aggressive. They don't like to be handled like most wild creatures, so of course caution is warranted.
When I find a snake in one of our coops, I use a long broom handle (a shepard's hook would be better) to remove it to an open area. A machete finishes the job. Make sure it's very sharp...snakes have tough hides!!
Black rat snakes are not poisonous. That’s almost certainly what that is. But they will eat eggs and chicks if they are big enough. Try to make your brooder as secure as you can. That can be challenging with snakes because they can get through some pretty small holes. Try to identify that snake. A poisonous snake is likely to be short and chunky while a black rat snake will be thinner and have a slender pencil-shaped head. Pretty sleek and attractive in my opinion.
I generally recommend against relocating any predator but I’m more relaxed about a black snake than any other predator. They do such a good job eating rodents and such. That’s probably why it is there. There are probably mice in the garage or the immediate vicinity. Snakes can handle being relocated better than practically any other predator without upsetting the ecosystem. Just make sure you take them a few miles so they don’t find their way back to their old territory.
Each state, city, and county have their own laws, but it’s pretty much illegal to release an animal anywhere other than on your own property. In many states it is illegal to kill a snake. I do whatever I feel is necessary but just to make you aware of that.
So what can you do other than try to secure your brooder? Deal with that snake, either relocating or killing it, whatever you decide. Then deal with the mice or whatever food source is drawing it to your garage and vicinity. Also, try to clean up piles of debris or stuff, inside and outside, that gives mice and snakes hiding places. I don’t do a good job on that last one.