chore: enforce shellcheck across the repo

Establish shellcheck as a mandatory pre-commit quality gate and bring all 93
shell scripts to a clean state.

- tests/shellcheck.sh: wrapper that runs koalaman/shellcheck:stable via Docker
  (no native binary needed), skips vendored + upstream librenms-agent scripts.
- .shellcheckrc: documents intentional codebase-wide disables (dynamic source
  paths SC1090/SC1091, client-side ssh expansion SC2029).
- AGENTS.md: new Git Policy rule mandating clean shellcheck for every shell
  script before commit.

Fixes applied (real bugs + quality): missing quote in netinfra/gather-configs.sh
(caused cascading parse errors), unquoted expansions, declare-and-assign masking,
egrep -> grep -E, $FUNCNAME array indexing, unused variable removal, cd || exit.
Intentional patterns (sourced config, sysfs/ps diagnostics, ssh heredocs that
expand local config) get justified targeted disables.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

netinfra/deep-audit-netboot.sh

@@ -37,9 +37,11 @@ for f in setupVars.conf pihole-FTL.conf adlists.list custom.list local.list rege
     echo "--- /etc/pihole/$f ---"; $DG exec pihole cat "/etc/pihole/$f" 2>&1 || true
 done
 echo "-- /etc/dnsmasq.d/* --"
+# shellcheck disable=SC2016 # $f/$t expand inside the container's sh, not locally — single quotes are intentional
 $DG exec pihole sh -c 'for f in /etc/dnsmasq.d/*; do echo "--- $f ---"; cat "$f"; done' 2>&1 || true
 echo "-- pihole version --";          $DG exec pihole pihole -v 2>&1 || true
 echo "-- gravity row counts --"
+# shellcheck disable=SC2016 # $t expands inside the container's sh, not locally
 $DG exec pihole sh -c 'for t in adlist domainlist client "group" info; do printf "%s=" "$t"; sqlite3 /etc/pihole/gravity.db "SELECT COUNT(*) FROM $t;" 2>/dev/null; done' 2>&1 || true
 echo "-- adlist addresses --"
 $DG exec pihole sqlite3 /etc/pihole/gravity.db "SELECT address,enabled,comment FROM adlist;" 2>&1 || true