cn commands: use omarchy-cmd helpers per bin style rule

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QKxGW1raAWaqeU8WdHsMsp

bin/omarchy-cn-ai-default

@@ -30,7 +30,7 @@ codex) agent="codex" ;;
 esac
 
 command=$(cn_ai_harness_field "$harness" command)
-if ! command -v "$command" > /dev/null 2>&1; then
+if omarchy-cmd-missing "$command"; then
   install=$(cn_ai_harness_field "$harness" install)
   echo "Installing $harness: $install"
   $install