Ssh keys work fine, if indicated in
./ssh/config but if i try
$ssh-add mykey even with
Could not open a connection to your authentication agent
ssh-add -l give the same result
ssh-add tries to add a key to the agent, but on your system no ssh-agent is running.
ssh has two different modes of working with keys. Either ssh is connecting to the agent and asking the agent to authenticate, or ssh is reading the key from disk.
As there is no agent running on your system the first mode is failing but the second one works, because there is no agent involved.
You can temporarily start an agent by
Usually the agent is started with your desktop environment. If you are not using a desktop environment you have to make sure by yourself that an agent is running if you need it.
If you are connected to a remote host by ssh. Then you might want to transfer your local agent to the remote host. Use