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

dns-cluster-setup/sync-zones.sh

@@ -35,7 +35,7 @@ mkdir -p "$ZONE_DIR"
 log "Syncing zones from $PRIMARY_HOST..."
 if rsync -az --delete --temp-dir=/tmp \
    "${PRIMARY_HOST}:$ZONE_DIR/" "$ZONE_DIR/" >> "$LOG_FILE" 2>&1; then
-  zone_count=$(ls "$ZONE_DIR" | wc -l)
+  zone_count=$(find "$ZONE_DIR" -maxdepth 1 -type f | wc -l)
   log "Sync complete: $zone_count zones"
 else
   log "ERROR: rsync failed (rc=$?)"