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.

extensions/ext.compose-ssh.yaml

@@ -12,5 +12,5 @@ capabilities:
   pricing_model: self-hosted
 config:
   base_dir: /tmp/sandboxer
-  ssh_user: root
+  # ssh_user omitted — uses SANDBOXER_SSH_USER or ~/.ssh/config (e.g. tegwick@coulombcore)
   compose_timeout_s: 180