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

k8s/verify.sh

@@ -99,7 +99,7 @@ echo "--- Workload isolation ---"
 USER_PODS=$(kubectl get pods -A --field-selector spec.nodeName="${CNODE1_NAME}" -o jsonpath='{.items[*].metadata.name}' 2>/dev/null | wc -w)
 # Subtract system pods
 SYSTEM_PODS=$(kubectl get pods -A --field-selector spec.nodeName="${CNODE1_NAME}" -l k8s-app --no-headers 2>/dev/null | wc -l)
-if [ "$USER_PODS" -le 10 ]; then ok "Only system pods on cnodes (expected)"; else fail "Unexpected pods on $CNODE1_NAME"; fi
+if [ "$((USER_PODS - SYSTEM_PODS))" -le 0 ]; then ok "Only system pods on cnodes (expected)"; else fail "Unexpected pods on $CNODE1_NAME"; fi
 
 echo ""
 echo "============================================"