feat(baoyu-youtube-transcript): add yt-dlp fallback and modularize codebase

Retry with alternate InnerTube client identities when YouTube returns
anti-bot responses, then fall back to yt-dlp when available. Split
monolithic main.ts into typed modules (youtube, transcript, storage,
shared, types) and add unit tests.

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

@@ -0,0 +1,60 @@
+import { existsSync, mkdirSync, readFileSync, writeFileSync } from "fs";
+import { dirname, join, resolve } from "path";
+
+import type { Sentence, Snippet, VideoMeta } from "./types.ts";
+
+export function ensureDir(path: string) {
+  const dir = dirname(path);
+  if (!existsSync(dir)) mkdirSync(dir, { recursive: true });
+}
+
+export function resolveBaseDir(outputDir: string): string {
+  return resolve(outputDir || "youtube-transcript");
+}
+
+function loadIndex(baseDir: string): Record<string, string> {
+  try {
+    return JSON.parse(readFileSync(join(baseDir, ".index.json"), "utf-8"));
+  } catch {
+    return {};
+  }
+}
+
+function saveIndex(baseDir: string, index: Record<string, string>) {
+  const path = join(baseDir, ".index.json");
+  ensureDir(path);
+  writeFileSync(path, JSON.stringify(index, null, 2));
+}
+
+export function lookupVideoDir(videoId: string, baseDir: string): string | null {
+  const rel = loadIndex(baseDir)[videoId];
+  if (rel) {
+    const dir = resolve(baseDir, rel);
+    if (existsSync(dir)) return dir;
+  }
+  return null;
+}
+
+export function registerVideoDir(videoId: string, channelSlug: string, titleSlug: string, baseDir: string): string {
+  const rel = join(channelSlug, titleSlug);
+  const index = loadIndex(baseDir);
+  index[videoId] = rel;
+  saveIndex(baseDir, index);
+  return resolve(baseDir, rel);
+}
+
+export function hasCachedData(videoDir: string): boolean {
+  return existsSync(join(videoDir, "meta.json")) && existsSync(join(videoDir, "transcript-raw.json"));
+}
+
+export function loadMeta(videoDir: string): VideoMeta {
+  return JSON.parse(readFileSync(join(videoDir, "meta.json"), "utf-8"));
+}
+
+export function loadSnippets(videoDir: string): Snippet[] {
+  return JSON.parse(readFileSync(join(videoDir, "transcript-raw.json"), "utf-8"));
+}
+
+export function loadSentences(videoDir: string): Sentence[] {
+  return JSON.parse(readFileSync(join(videoDir, "transcript-sentences.json"), "utf-8"));
+}