refactor(cli): clarify topic folder handling and remove unused groupFolder helper

apps/cli/sync.ts

@@ -12,11 +12,17 @@
  *   render as a plain fence.
  *
  * Two classifications, two homes. The sidebar group is the campaign TOPIC
- * from the README table (`(tree-dfs)/`, ordered by topic number via a
+ * from the README table (`tree-dfs/`, ordered by topic number via a
  * script-owned `meta.ts`); the page badge is the data-structure CATEGORY the
  * file sits under in `work/1/<Difficulty>/<Category>/`. A problem outside the
  * plan (warm-ups) has no topic, so it groups under its category instead.
  *
+ * Topic folders are real route segments (`/tree-dfs/invert-binary-tree`), not
+ * `(group)` folders, so a hand-written `index.mdx` in one is the topic's
+ * landing page at `/tree-dfs` and Blume links the sidebar group row to it.
+ * Sync never writes or moves an `index.mdx`, and never removes a folder that
+ * still holds one.
+ *
  * Only the first-solve bucket is read: work/3 and work/7 hold blind re-solves
  * of problems whose page already exists (see work.ts), and a fresh scaffold
  * there would otherwise wipe the page's `## Solution`.
@@ -190,11 +196,6 @@ async function readmeTopics(): Promise<Map<number, Topic>> {
   return map;
 }
 
-/** `(group)` folder name: route-transparent in Blume, so only the sidebar sees it. */
-function groupFolder(topic: Topic): string {
-  return `(${topic.name.toLowerCase().replace(/[^a-z0-9]+/g, "-").replace(/^-|-$/g, "")})`;
-}
-
 /** Blume folder meta: exact topic label (the folder slug loses `/` and case) and curriculum order. */
 function groupMeta(topic: Topic): string {
   const order = topic.order === undefined ? "" : `, order: ${topic.order}`;
@@ -422,11 +423,12 @@ const vacated = new Set<string>(); // folders a page moved out of; removed below
 for (const sol of ordered) {
   const langs = Object.keys(sol.code).sort().join("+");
   const workRef = `${sol.num}.${sol.slug} (${langs})`;
-  // (topic) group folder + leetcode-number prefix: numeric sidebar order, both stripped from the URL.
+  // <topic>/ folder (a real route segment, so its index.mdx is the topic landing
+  // page) + leetcode-number prefix: numeric sidebar order, prefix stripped from the URL.
   // An unplanned problem's category may share a folder with a numbered topic
   // (`Hash Table` warm-ups under topic 1); the numbered one owns the meta.
   const topic = topics.get(sol.num) ?? { name: sol.category };
-  const group = groupFolder(topic);
+  const group = topic.name.toLowerCase().replace(/[^a-z0-9]+/g, "-").replace(/^-|-$/g, "");
   if (groups.get(group)?.order === undefined) groups.set(group, topic);
   const name = `${group}/${sol.num}-${sol.slug}.mdx`;
   const existing = pages.get(sol.num);