ssh: add GitHub SSH key and host config

ssh/app-github.pub

@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIN9f2p8HTbjdo4GJHusMa8jtjSIcOrTmL6nxpvnUZVcq

ssh/config

@@ -8,6 +8,11 @@ Host git.mitsea.com
     User git
     IdentityFile ~/.flinty/ssh/app-gitea-selfhost.pub
 
+Host github.com
+    HostName github.com
+    User git
+    IdentityFile ~/.flinty/ssh/app-github.pub
+
 Host inc-r750xa
     HostName 10.0.24.20
     User ubuntu