In theory, enabling SSH support on the `gpg-agent` is as simple as passing the `--enable-ssh-support` option to the `gpg-agent`
when initialized. However, this is tricky in Ubuntu (and probably many other Linux systems) because the `gpg-agent` is
constantly relaunched. If we're on a machine we don't want to modify too much (as in disabling the classic `ssh-agent`), this
is even trickier.

Here's how I solved it:

* Enable SSH support for the `gpg-agent` every time. While this is a system modification, it's a mild one and easy to revert.
  This is accomplished with the command:

```sh
echo enable-ssh-support >> .gnupg/gpg-agent.conf
```

* Then, kill the `gpg-agent` manually. It'll be restarted automatically and pick up the new option:

```sh
kill $(pgrep gpg-agent)
```

* Finally, have the SSH client point to the `gpg-agent` when looking for keys, instead of the traditional `ssh-agent`. This
  will only affect the current shell, so it won't affect the system beyond that. It's accomplished by rewiring the following
  environment variable:

```sh
export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket)
```

And that's it! If you have an authentication GPG subkey on a smartcard, you can now use that to connect through SSH.