refactor: strip DECNET tokens from container-visible surface

Rename the container-side logging module decnet_logging → syslog_bridge
(canonical at templates/syslog_bridge.py, synced into each template by
the deployer). Drop the stale per-template copies; setuptools find was
picking them up anyway. Swap useradd/USER/chown "decnet" for "logrelay"
so no obvious token appears in the rendered container image.

Apply the same cloaking pattern to the telnet template that SSH got:
syslog pipe moves to /run/systemd/journal/syslog-relay and the relay
is cat'd via exec -a "systemd-journal-fwd". rsyslog.d conf rename
99-decnet.conf → 50-journal-forward.conf. SSH capture script:
/var/decnet/captured → /var/lib/systemd/coredump (real systemd path),
logger tag decnet-capture → systemd-journal. Compose volume updated
to match the new in-container quarantine path.

SD element ID shifts decnet@55555 → relay@55555; synced across
collector, parser, sniffer, prober, formatter, tests, and docs so the
host-side pipeline still matches what containers emit.

decnet/logging/syslog_formatter.py

@@ -5,7 +5,7 @@ Produces fully-compliant syslog messages:
   <PRI>1 TIMESTAMP HOSTNAME APP-NAME PROCID MSGID [SD-ELEMENT] MSG
 
 Facility: local0 (16)
-PEN for structured data: decnet@55555
+PEN for structured data: relay@55555
 """
 
 from __future__ import annotations
@@ -16,7 +16,7 @@ from typing import Any
 
 FACILITY_LOCAL0 = 16
 NILVALUE = "-"
-_SD_ID = "decnet@55555"
+_SD_ID = "relay@55555"
 
 SEVERITY_INFO    = 6
 SEVERITY_WARNING = 4