fix(ssh-agent): add error message if `~/.ssh` is not found (#11929)

Closes #11829 

Co-authored-by: Carlo Sala <carlosalag@protonmail.com>

plugins/ssh-agent/ssh-agent.plugin.zsh

@@ -13,6 +13,11 @@ function _start_agent() {
     fi
   fi
 
+  if [[ ! -d "$HOME/.ssh" ]]; then
+    echo "[oh-my-zsh] ssh-agent plugin requires ~/.ssh directory"
+    return 1
+  fi
+
   # Set a maximum lifetime for identities added to ssh-agent
   local lifetime
   zstyle -s :omz:plugins:ssh-agent lifetime lifetime