refactor(content-guards): replace fragile bash expansion with explicit branching

The previous fix (PR #39, commit 2202144) used ${config_flag[@]+"${config_flag[@]}"} to work around
bash 3.2's unbound variable error with empty arrays. While functional, this expansion is:
- Hard to read and understand
- Easy to get wrong (even the commit message described it backwards)
- Fragile across bash versions

This change replaces the arcane parameter expansion with explicit conditional branching:
- Check array length with ${#config_flag[@]} (works correctly under set -u)
- Separate code paths for empty/non-empty arrays
- Clear, maintainable, and works on all bash versions including macOS bash 3.2

Verified with /bin/bash (bash 3.2) under set -euo pipefail with both code paths.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: JacobPEvans

content-guards/scripts/validate-markdown.sh

@@ -104,7 +104,12 @@ EOF
     fi
   fi
 
-  if ! markdownlint_output=$(markdownlint-cli2 "${config_flag[@]+"${config_flag[@]}"}" "$file_path" 2>&1); then
+  if [[ ${#config_flag[@]} -gt 0 ]]; then
+    markdownlint_output=$(markdownlint-cli2 "${config_flag[@]}" "$file_path" 2>&1)
+  else
+    markdownlint_output=$(markdownlint-cli2 "$file_path" 2>&1)
+  fi
+  if [[ $? -ne 0 ]]; then
     errors+=("markdownlint-cli2 failed:")
     errors+=("$markdownlint_output")
   fi