docs: recommend official Homebrew cask installation

- Remove personal tap requirement from all READMEs (en/zh/ja)
- Update v3.15.0 release notes to highlight official cask availability
- Add celebration banner noting Homebrew official repository inclusion
- Simplify installation to single command: brew install --cask cc-switch

docs/release-notes/v3.15.0-en.md

@@ -499,8 +499,9 @@ Visit [Releases](https://github.com/farion1231/cc-switch/releases/latest) to dow
 
 ### Homebrew (macOS)
 
+> 🎉 CC Switch is now available in the official Homebrew cask repository — no need to add a custom tap!
+
 ```bash
-brew tap farion1231/ccswitch
 brew install --cask cc-switch
 ```
 

docs/release-notes/v3.15.0-ja.md

@@ -499,8 +499,9 @@ Usage サマリーはキャッシュ正規化後の真の総トークン数 +
 
 ### Homebrew（macOS）
 
+> 🎉 CC Switch は Homebrew 公式 cask リポジトリに収録されました。カスタム tap の追加は不要です！
+
 ```bash
-brew tap farion1231/ccswitch
 brew install --cask cc-switch
 ```
 

docs/release-notes/v3.15.0-zh.md

@@ -499,8 +499,9 @@ Release notes 模板中呈现 `ccswitch.io`。
 
 ### Homebrew（macOS）
 
+> 🎉 CC Switch 现已收录至 Homebrew 官方 cask 仓库，无需添加第三方 tap！
+
 ```bash
-brew tap farion1231/ccswitch
 brew install --cask cc-switch
 ```