Parse RFC 4253 §4.2 identification strings from the first attacker→decky
data segment on TCP/22; emit ssh_client_banner syslog events and bus
fan-out. Profiler's sniffer_rollup dedupes observed banners into a new
AttackerBehavior.ssh_client_banners JSON column.
Closes gap #3 from SIGNAL_CAPTURE_AUDIT.md.