docs: document Kugou (酷狗音乐) source + login features in README

Kugou shipped in #110 but the README still listed only netease/qq/bilibili/
youtube. Add Kugou throughout:
- tagline, badge, 多平台音源, QR 登录, 歌单管理 (酷狗私人电台 !fm -k + the
  login-gated daily/recommend/user playlists)
- quick-start account login, WebUI page table (FM sources, 三→四平台 search,
  multi-platform login), architecture tree (kugou.ts), dependency table,
  milestones, and a credit to the MIT MakcRe/KuGouMusicApi reference
- command table: !play -k / !search [-k] / !artist -k / !fm -k (the flags that
  actually route to Kugou; not !playlist -k — Kugou search returns no playlists)

Also fix the in-bot `!search` usage string to include -k so it matches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/bot/instance.ts

@@ -654,7 +654,7 @@ export class BotInstance extends EventEmitter {
 
   private async cmdSearch(cmd: ParsedCommand): Promise<string> {
     const p = this.config.commandPrefix;
-    if (!cmd.args) return `Usage: ${p}search <name> [-q|-b|-y]`;
+    if (!cmd.args) return `Usage: ${p}search <name> [-q|-k|-b|-y]`;
     const provider = this.getProvider(cmd.flags);
     const result = await provider.search(cmd.args, 8);
     if (result.songs.length === 0) return `No results found for: ${cmd.args}`;