swarm-otel: ship the whole host journal, drop user sessions after it

The swarm collector's journald receiver read only the units listed in
`services.hyperhive.swarm.otel.journaldUnits`. A unit nobody listed
never reached the store, and a misspelt entry shipped nothing without
an error. The list existed to keep an operator's desktop session out of
a store every swarm operator can read, but the receiver can only match
positively, so the only way to express "not user sessions" was to name
every service instead.

The receiver now reads the whole host journal, and a new
`filter/exclude-user-sessions` processor in the `logs/<swarm>` pipeline
drops records whose `_SYSTEMD_SLICE` is `user-<uid>.slice` (session
scopes and `user@<uid>.service`). The per-hive `logs/<hive>` pipelines
carry agent-container journals only and get no filter.

`journaldUnits` is removed with `mkRemovedOptionModule`, together with
its non-empty assertion and the entry each host module added. The four
module-eval membership checks go with it, replaced by one structural
case in swarm-otel-core.

Closes #3646

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

@@ -466,14 +466,6 @@ in
   };
 
   config = lib.mkIf deployCfg.forgejo.enable {
-    # Same principle as the vhost below — this service's own surface lives
-    # with the service. The SSO source is named alongside forgejo because
-    # its failure mode is a login that silently falls back, not an error.
-    services.hyperhive.swarm.otel.journaldUnits = [
-      "forgejo"
-      "forgejo-sso-source"
-    ];
-
     # This service's own gateway surface: the vhost that fronts it and
     # the name the hive resolver answers for. Declared here rather than
     # in the gateway so the forge's public face lives with the forge —