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>

decnet/services/real_ssh.py

@@ -0,0 +1,46 @@
+from pathlib import Path
+
+from decnet.services.base import BaseService
+
+TEMPLATES_DIR = Path(__file__).parent.parent.parent / "templates" / "real_ssh"
+
+
+class RealSSHService(BaseService):
+    """
+    Fully interactive OpenSSH server — no honeypot emulation.
+
+    Used for the deaddeck (entry-point machine). Attackers get a real shell.
+    Credentials are intentionally weak to invite exploitation.
+
+    service_cfg keys:
+        password      Root password (default: "admin")
+        hostname      Override container hostname
+    """
+
+    name = "real_ssh"
+    ports = [22]
+    default_image = "build"
+
+    def compose_fragment(
+        self,
+        decky_name: str,
+        log_target: str | None = None,
+        service_cfg: dict | None = None,
+    ) -> dict:
+        cfg = service_cfg or {}
+        env: dict = {
+            "SSH_ROOT_PASSWORD": cfg.get("password", "admin"),
+        }
+        if "hostname" in cfg:
+            env["SSH_HOSTNAME"] = cfg["hostname"]
+
+        return {
+            "build": {"context": str(TEMPLATES_DIR)},
+            "container_name": f"{decky_name}-real-ssh",
+            "restart": "unless-stopped",
+            "cap_add": ["NET_BIND_SERVICE"],
+            "environment": env,
+        }
+
+    def dockerfile_context(self) -> Path:
+        return TEMPLATES_DIR