Add deaddeck: real interactive SSH entry-point machine

Introduces the 'real_ssh' service plugin backed by a genuine OpenSSH
server (not cowrie), and the 'deaddeck' archetype that uses it. The
container ships with a lived-in Linux environment and a deliberately
weak root:admin credential to invite exploitation.

- templates/real_ssh/: Dockerfile + entrypoint (configurable via env)
- decnet/services/real_ssh.py: BaseService plugin, service_cfg supports
  password and hostname overrides
- decnet/archetypes.py: deaddeck archetype added
- tests/test_real_ssh.py: 17 tests covering registration, compose
  fragment structure, overrides, and archetype

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/real_ssh/entrypoint.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+set -e
+
+# Configure root password (default: admin)
+ROOT_PASSWORD="${SSH_ROOT_PASSWORD:-admin}"
+echo "root:${ROOT_PASSWORD}" | chpasswd
+
+# Optional: override hostname inside container
+if [ -n "$SSH_HOSTNAME" ]; then
+    echo "$SSH_HOSTNAME" > /etc/hostname
+    hostname "$SSH_HOSTNAME"
+fi
+
+# Generate host keys if missing (first boot)
+ssh-keygen -A
+
+# Fake bash history so the box looks used
+if [ ! -f /root/.bash_history ]; then
+    cat > /root/.bash_history <<'HIST'
+apt update && apt upgrade -y
+systemctl status nginx
+tail -f /var/log/syslog
+df -h
+htop
+ps aux | grep python
+git pull origin main
+cd /root/projects
+vim notes.txt
+crontab -e
+ls /var/www/html
+HIST
+fi
+
+exec /usr/sbin/sshd -D -e