Fix SSH auth: respect ~/.ssh/config instead of defaulting to root

CoulombCore (92.205.130.254) authenticates as tegwick via id_ops in
ssh config, not root. Omit SANDBOXER_SSH_USER to let OpenSSH apply config;
set SANDBOXER_SSH_USER only to override.

tests/test_ssh.py

@@ -0,0 +1,13 @@
+"""SSH config behavior."""
+
+from sandboxer.extensions.ssh import SSHConfig
+
+
+def test_destination_uses_ssh_config_when_user_unset() -> None:
+    ssh = SSHConfig(host="92.205.130.254")
+    assert ssh.destination == "92.205.130.254"
+
+
+def test_destination_includes_explicit_user() -> None:
+    ssh = SSHConfig(host="92.205.130.254", user="tegwick")
+    assert ssh.destination == "tegwick@92.205.130.254"