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-gateway/dns.nix

@@ -30,10 +30,6 @@ in
     # address, so the resolver is itself a consumer one layer down.
     services.hyperhive.network.enable = lib.mkDefault true;
 
-    # A name that stops resolving presents as every client timing out at
-    # once, so this unit's journal is worth reading swarm-wide.
-    services.hyperhive.swarm.otel.journaldUnits = [ "dnsmasq" ];
-
     # The host asks the hive's own resolver, at the BRIDGE IP.
     #
     # Every container inherits a COPY of this host's `/etc/resolv.conf`