otel: ship the journals the dashboard can already show

The dashboard's log surface is an allow-list: the four harness units in
every agent container, the hive-ci / hive-forge / hive-matrix machines,
and three host units. `swarm.otel.journaldUnits` only named the host
three plus forgejo — so the units an operator can read in the UI were
absent from the store they are supposed to be queryable in, and nothing
reports that, because a name that matches nothing is not an error.

Adds the four agent units, the CI runner and the homeserver. The agent
units are declared by the hive-c0re module rather than by the
agent-modules that define them: those are a guest evaluation and cannot
reach a host option.

Closes #3818

nix/host-modules/hive-ci.nix

@@ -176,6 +176,11 @@ in
       }
     ];
 
+    # The runner's journal, named as the unit is *inside* the container —
+    # nixpkgs derives `gitea-runner-<instance>` from the attr below, so this
+    # name follows that attr rather than `cfg.name`.
+    services.hyperhive.swarm.otel.journaldUnits = [ "gitea-runner-hive" ];
+
     # Create /run/hive-ci/ on the host and seed runner-token with a
     # placeholder. The container bind-mounts this file read-only; hive-c0re
     # (via hive-priv's RegisterCiRunner) overwrites it with the real