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/collector/worker.py

@@ -114,7 +114,7 @@ _RFC5424_RE = re.compile(
     r"(\S+) "       # 4: MSGID (event_type)
     r"(.+)$",       # 5: SD element + optional MSG
 )
-_SD_BLOCK_RE = re.compile(r'\[decnet@55555\s+(.*?)\]', re.DOTALL)
+_SD_BLOCK_RE = re.compile(r'\[relay@55555\s+(.*?)\]', re.DOTALL)
 _PARAM_RE = re.compile(r'(\w+)="((?:[^"\\]|\\.)*)"')
 _IP_FIELDS = ("src_ip", "src", "client_ip", "remote_ip", "remote_addr", "target_ip", "ip")