fix: backfill chapter end times for cached videos

Videos cached before the chapter end-time change would silently
lack the 'end' field when loaded from cache. This adds a migration
that detects missing 'end' fields on cache hit, computes them from
adjacent chapters, and persists the updated meta.json.

This ensures consistent output regardless of whether the data was
freshly fetched or loaded from cache.

skills/baoyu-youtube-transcript/scripts/main.ts

@@ -699,6 +699,15 @@ async function processVideo(videoId: string, opts: Options): Promise<VideoResult
     sentences = loadSentences(videoDir);
     const wantLangs = opts.translate ? [opts.translate] : opts.languages;
     if (!wantLangs.includes(meta.language.code)) needsFetch = true;
+    // Backfill chapter end times for caches created before this field existed
+    if (meta.chapters.length > 0 && meta.chapters[0].end === undefined) {
+      for (let i = 0; i < meta.chapters.length; i++) {
+        (meta.chapters[i] as any).end = i < meta.chapters.length - 1
+          ? meta.chapters[i + 1].start
+          : meta.duration;
+      }
+      writeFileSync(join(videoDir, "meta.json"), JSON.stringify(meta, null, 2));
+    }
   }
 
   if (needsFetch) {