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-tls.nix

@@ -420,16 +420,6 @@ in
     # rebuild of every hive that is not the CA host, about a fallback
     # that no longer happens.
 
-    # The first two run on the deploy and sit on the gateway's start path,
-    # so a failure in either takes TLS down for every service behind it.
-    # The renewal sits on no start path, and a failure there is a leaf that
-    # lapses days later unless someone reads why it failed now.
-    services.hyperhive.swarm.otel.journaldUnits = [
-      "hive-tls-ca"
-      "swarm-services-cert"
-      "swarm-services-cert-renew"
-    ];
-
     # Generate (and rotate) the hive CA + gateway leaf before anything
     # serves it. Idempotent: the CA is created once and reused; the leaf
     # is re-signed on expiry under the same CA so the anchor is stable.