ssh: add github ssh key config and setup script

.gitignore

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+# Ignore SSH known hosts
+known_hosts
+known_hosts.old
+# Ignore SSH keys since they are stored and managed by
+# the Yubikey.
+id_ed25519_sk_rk_*

.ssh/config

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+Host github.com
+    HostName github.com
+    User git
+    IdentityFile ~/.ssh/id_ed25519_sk_rk_servostar@github.com

run-setup-ssh.sh

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+#!/bin/sh
+
+# Retrieve key from Yubikey
+echo "==> Getting keys from Yubikey..."
+cd .ssh
+ssh-keygen -K
+