docs: add npx alternative and Snap Bun warning for Ubuntu users (#401)

Closes #389

- Add npx as alternative when bunx doesn't work
- Add warning about Snap-installed Bun not working with bunx
- Update all localized READMEs (ko, ja, zh-cn) with same changes

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

README.zh-cn.md

@@ -231,8 +231,12 @@ fi
 
 ```bash
 bunx oh-my-opencode install --no-tui --claude=<yes|no|max20> --chatgpt=<yes|no> --gemini=<yes|no>
+# 如果 bunx 不好使就换 npx
+npx oh-my-opencode install --no-tui --claude=<yes|no|max20> --chatgpt=<yes|no> --gemini=<yes|no>
 ```
 
+> **Ubuntu/Debian 用户注意**：如果你是用 Snap 装的 Bun (`/snap/bin/bun`)，由于 Snap 的沙箱机制，`bunx` 会报 "script not found" 错误。要么改用 `npx`，要么用官方脚本重装 Bun：`curl -fsSL https://bun.sh/install | bash`
+
 **例子：**
 - 全套订阅 + max20：`bunx oh-my-opencode install --no-tui --claude=max20 --chatgpt=yes --gemini=yes`
 - 只有 Claude（没 max20）：`bunx oh-my-opencode install --no-tui --claude=yes --chatgpt=no --gemini=no`