Chimney on 100 year old home has pulled away from outside wall

You didn't mention how tall the chimney is. 2 story house or single. Is there or was there ever a displacement for a fireplace, one flue or two? Lined or bare brick interior?

In general, the procedure is brick by brick. A small mini jack/impact hammer will speed up the process, but it can be done with a hammer and cold chisel. Most chimneys are independently supported and not structural to walls or the roof. there may be ties to framing members that you will have to cut or remove as you go through the roof or floor. You definitely want to use basic safety equipment like a dust mask, gloves and good eye protection. You can cut down on the dust and debris spreading in the house by hanging plastic from the ceilings around the work area. Use some painter's tape to hang lightweight plastic from the ceiling creating an enclosed area around your work area.

One cool trick on chimneys with a large flue opening and a clean out on the bottom is to drop the bricks one at a time to the bottom and have a helper remove them and stack them at the bottom. This is a bit neater and saves bringing bricks through the house.

This is common but its also a big issue. I doubt that this has to do with your house foundation although it could be a factor. The main cause would be a weak footing below your chimney.

The fact that you are having a dry year might have caused the footings to move earlier but if they weren't up to spec then it was going to move. You have to figure that your chimney may weigh a few tons. It has the house on one side. So the chimney is going to move away from pressure if the footings are weak. So that is why the top is separating probably a little more than the bottom.

I am not sure exactly who was supposed to do what to make the chimney 7 years ago.... But normally the mason would create proper footings or at least not build until it was done right. If your mason was contracted to do everything he messed up or your soil is completely unstable (which he should have researched before doing the job).

So now you need to get this fixed ASAP. If it hasn't had too much damage they can jack it back into place, repair footings, and bracket it into place. This is much much cheaper than tear down and build. If you have a warranty you will need to keep trying to contact the mason - maybe he gives you a deal on the repair. Fixing this yourself really isn't DIY because the tools/equipment are too costly for a one time job.