docs: AGENTS.md repoint to KNEL repos + stale-ref fixes [#769]

Completes dba54b2 (which staged deletions only — pathspec'd add missed
the content edits). Layout table, Key Scripts, DNS discipline, conman
section now point at KNEL/netinfra|facilities|perf|monitoring|inventory;
conman example switched from IP literal to pfv-tsys4 (DNS-names rule).

https://projects.knownelement.com/issues/769#note-4152

tests/shellcheck.sh

@@ -24,7 +24,6 @@ mapfile -t FILES < <(
       case "$f_abs" in
         "$ROOT"/vendor/*) ;;                              # skip vendored
         "$ROOT"/archive/provisioning/Agents/librenms/*) ;; # upstream librenms-agent scripts
-        "$ROOT"/oam/librenms-agent/agent/*) ;;             # upstream librenms-agent scripts (verbatim port [#474])
         "$ROOT"/archive/KNELServerBuild/*) ;;              # archived legacy code, not maintained [#474]
         *) case "$f_abs" in *.sh) echo "$f_abs";; esac ;;
       esac
@@ -35,7 +34,6 @@ mapfile -t FILES < <(
       case "$f_abs" in
         "$ROOT"/vendor/*) ;;                              # skip vendored
         "$ROOT"/archive/provisioning/Agents/librenms/*) ;; # upstream librenms-agent scripts
-        "$ROOT"/oam/librenms-agent/agent/*) ;;             # upstream librenms-agent scripts (verbatim port [#474])
         "$ROOT"/archive/KNELServerBuild/*) ;;              # archived legacy code, not maintained [#474]
         *) echo "$f_abs" ;;
       esac