ci: add shellcheck lint workflow; fix 6 violations

Uses \$'...' quoting for spinner backslash (SC1003). Adds source=/dev/null
for /etc/os-release (SC1091). Splits local+assign for ip and
current_version (SC2155). Adds disable comments for intentional literal
regex matches (SC2076).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

proxDoc.sh

@@ -150,7 +150,7 @@ unit_file_exists() {
 wait_for_process() {
     local pid="$1"
     local delay="${2:-0.1}"
-    local spinner='|/-\'
+    local spinner=$'|/-\\'
     local i=0
 
     while kill -0 "$pid" 2>/dev/null; do
@@ -177,6 +177,7 @@ get_system_info() {
     else
         echo -e "\n${GREEN}=== OS Information ===${NC}"
         if [[ -f /etc/os-release ]]; then
+            # shellcheck source=/dev/null
             source /etc/os-release
             echo -e "${GREEN}Distribution:${NC} $PRETTY_NAME"
         else
@@ -545,7 +546,8 @@ get_node_exporter_status() {
     echo -e "\n${GREEN}=== Node Exporter Status ===${NC}"
     if systemctl is-active --quiet node_exporter 2>/dev/null; then
         echo -e "${GREEN}Service:${NC} Running"
-        local ip=$(hostname -I | awk '{print $1}')
+        local ip
+        ip=$(hostname -I | awk '{print $1}')
         echo -e "${GREEN}Metrics URL:${NC} http://${ip}:9100/metrics"
         if ss -tlnp 2>/dev/null | grep -q ':9100'; then
             echo -e "${GREEN}Port 9100:${NC} Listening"
@@ -589,6 +591,7 @@ run_selective_checks() {
     # Validate check names
     IFS=',' read -ra check_array <<< "$checks"
     for check in "${check_array[@]}"; do
+        # shellcheck disable=SC2076  # intentional literal match (not regex)
         if [[ ! " $VALID_CHECKS " =~ " $check " ]]; then
             log_message error "Unknown check: $check"
             echo "Valid checks: $VALID_CHECKS"
@@ -675,7 +678,8 @@ check_services() {
 
 check_pve_version() {
     local min_version="6.0"
-    local current_version=$(pveversion | grep -oP 'pve-manager/\K[0-9]+\.[0-9]+' || echo "0.0")
+    local current_version
+    current_version=$(pveversion | grep -oP 'pve-manager/\K[0-9]+\.[0-9]+' || echo "0.0")
     if (( $(echo "$current_version < $min_version" | bc -l) )); then
         log_message warn "Proxmox VE version $current_version may not support all features"
     fi
@@ -872,7 +876,7 @@ validate_input() {
     fi
     # Extract the option name (before any = sign)
     local opt_name="${input%%=*}"
-    # Check against whitelist
+    # shellcheck disable=SC2076  # intentional literal match (not regex)
     if [[ ! " $VALID_OPTIONS " =~ " $opt_name " ]]; then
         return 1
     fi