Why does ssh-agent fail and do I need to add the key again via ssh-add (Debian 8x64)?

Question

SSH

Hello!
There is a VDS on Debian 8 x64, I use it to test projects (Laravel / Flask) before uploading to the client. I use GitHub with a paid annual subscription (to be able to create private repositories).
So, every time I go to the server via SSH to roll new changes in the project, I have to first execute eval "$(ssh-agent -s)"(to check the pid of the agent), then add the RSA key ssh-add ~/.ssh/rsa_project_key, and only after that it allows me to do git pull..
If this is not done, then it git pulloutputs:

Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

And so every time you start a new SSH session. Maybe someone came across?
PS I did the initial setup according to this manual: automagically.ru/privatnyiy-git-server-vps Maybe there is a more correct option for setting up private repositories on VDS?