fix: guard --output-dir arg and use configurable ZOMBIE_WARN threshold

- Validate --output-dir has a following argument before accessing $2
  (prevents unbound variable crash under set -u)
- Add ZOMBIE_WARN config variable (default: 1) and use it in the zombie
  check instead of hardcoding 0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

monitoring/scripts/homelab-audit.sh

@@ -27,10 +27,15 @@ DISK_WARN=80
 DISK_CRIT=90
 LOAD_WARN=2.0
 MEM_WARN=85
+ZOMBIE_WARN=1
 
 while [[ $# -gt 0 ]]; do
   case "$1" in
     --output-dir)
+      if [[ $# -lt 2 ]]; then
+        echo "Error: --output-dir requires an argument" >&2
+        exit 1
+      fi
       REPORT_DIR="$2"
       shift 2
       ;;
@@ -179,7 +184,7 @@ parse_and_report() {
         ;;
       ZOMBIES=*)
         local zombies="${line#ZOMBIES=}"
-        if [[ -n "$zombies" ]] && ((zombies > 0)); then
+        if [[ -n "$zombies" ]] && ((zombies >= ZOMBIE_WARN)); then
           echo "WARN  $label: ${zombies} zombie process(es)" >>"$FINDINGS_FILE"
         fi
         ;;