swarm-otel: collect only the units the swarm's services declare

The journald receiver was configured with a directory and no filter, so
the swarm's log store received every unit on the host that runs the
collector. On a hive whose services live on a workstation that includes
the operator's desktop session, in a store every swarm operator can read.

The receiver has no system-only switch and its `matches` field is an
allowlist too, so what the swarm collects has to be stated rather than
excluded. Each service module names its own units: a service that is not
running contributes nothing, and one added later arrives declared.

An empty list is fail-open — the receiver renders no filter at all and
reads everything — so it is asserted against.

nix/host-modules/hive-c0re/default.nix

@@ -136,6 +136,14 @@ in
   ];
 
   config = lib.mkIf cfg.enable {
+    # The daemon that owns every container on this hive, and the helper it
+    # delegates its root operations to. An agent asking why a container did
+    # not come up is asking about one of these two.
+    services.hyperhive.swarm.otel.journaldUnits = [
+      "hive-c0re"
+      "hive-priv"
+    ];
+
     assertions = [
       {
         # The pinned claude reaches agents as a bare path, so this