feat(swarm): DeckyConfig.host_uuid + fix agent log/status field refs

- decnet.models.DeckyConfig grows an optional 'host_uuid' (the SwarmHost
  that runs this decky). Defaults to None so legacy unihost state files
  deserialize unchanged.
- decnet.agent.executor: replace non-existent config.name references
  with config.mode / config.interface in logs and status payload.
- tests/swarm/test_state_schema.py covers legacy-dict roundtrip, field
  default, and swarm-mode assignments.

tests/swarm/test_state_schema.py

@@ -0,0 +1,60 @@
+"""Backward-compatibility tests for the SWARM state-schema extension.
+
+DeckyConfig gained an optional ``host_uuid`` field in swarm mode.  Existing
+state files (unihost) must continue to deserialize without change.
+"""
+from __future__ import annotations
+
+from decnet.models import DeckyConfig, DecnetConfig
+
+
+def _minimal_decky(name: str = "decky-01") -> dict:
+    return {
+        "name": name,
+        "ip": "192.168.1.10",
+        "services": ["ssh"],
+        "distro": "debian",
+        "base_image": "debian:bookworm-slim",
+        "hostname": "decky01",
+    }
+
+
+def test_decky_config_host_uuid_defaults_to_none() -> None:
+    """A decky built from a pre-swarm state blob lands with host_uuid=None."""
+    d = DeckyConfig(**_minimal_decky())
+    assert d.host_uuid is None
+
+
+def test_decky_config_accepts_host_uuid() -> None:
+    d = DeckyConfig(**_minimal_decky(), host_uuid="host-uuid-abc")
+    assert d.host_uuid == "host-uuid-abc"
+
+
+def test_decnet_config_mode_swarm_with_host_assignments() -> None:
+    """Full swarm-mode config: every decky carries a host_uuid."""
+    config = DecnetConfig(
+        mode="swarm",
+        interface="eth0",
+        subnet="192.168.1.0/24",
+        gateway="192.168.1.1",
+        deckies=[
+            DeckyConfig(**_minimal_decky("decky-01"), host_uuid="host-A"),
+            DeckyConfig(**_minimal_decky("decky-02"), host_uuid="host-B"),
+        ],
+    )
+    assert config.mode == "swarm"
+    assert {d.host_uuid for d in config.deckies} == {"host-A", "host-B"}
+
+
+def test_legacy_unihost_state_still_parses() -> None:
+    """A dict matching the pre-swarm schema deserializes unchanged."""
+    legacy_blob = {
+        "mode": "unihost",
+        "interface": "eth0",
+        "subnet": "192.168.1.0/24",
+        "gateway": "192.168.1.1",
+        "deckies": [_minimal_decky()],
+    }
+    config = DecnetConfig.model_validate(legacy_blob)
+    assert config.mode == "unihost"
+    assert config.deckies[0].host_uuid is None