refactor(cli): default group order to 0 and clarify meta comment

apps/cli/sync.ts

@@ -196,10 +196,13 @@ async function readmeTopics(): Promise<Map<number, Topic>> {
   return map;
 }
 
-/** Blume folder meta: exact topic label (the folder slug loses `/` and case) and curriculum order. */
+/**
+ * Blume folder meta: exact topic label (the folder slug loses `/` and case) and
+ * curriculum order. An unplanned category group (warm-ups) has no README
+ * number and sorts at 0, ahead of topic 1, so the sidebar opens on the basics.
+ */
 function groupMeta(topic: Topic): string {
-  const order = topic.order === undefined ? "" : `, order: ${topic.order}`;
+  return `import { defineMeta } from "blume";\n\nexport default defineMeta({ title: ${JSON.stringify(topic.name)}, order: ${topic.order ?? 0} });\n`;
-  return `import { defineMeta } from "blume";\n\nexport default defineMeta({ title: ${JSON.stringify(topic.name)}${order} });\n`;
 }
 
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