Using SSH Git: key lost after system restart?

I am setting up Git usage on Raspbian. This worked OK:

cd .ssh
   ssh-keygen -t rsa -b 4096 -C "name@asdf.com"
   eval $(ssh-agent -s)
   ssh-add ./id_rsa_rpi

After I created the keys locally, I went to GitHub and added a public key there. It all worked OK, I cloned a few repositories without problems. Confirmed it all with ssh -vT git@github.com However, after a shutdown yesterday and a powerup today, git pull didn't work: git@github.com: Permission denied (publickey). I had to repeat the following:

eval $(ssh-agent -s)
ssh-add .ssh/id_rsa_rpi

Now it works again. How do I make this permanent?