fish: set up SSH socket management on Andoid

fish/config.fish

···
     command -qs zoxide; and zoxide init fish | source
 end
 
+# If we're in Termux, add the SSH agent
+if uname -a | grep -q Android
+    set -x SSH_AUTH_SOCK $PREFIX/var/run/ssh-agent.socket
+    ssh-add
+end
+
 # General
 set -x BAT_THEME "Monokai Extended Bright"
 set -x COLORTERM truecolor