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.md

@@ -204,8 +204,12 @@ Run the interactive installer:
 
 ```bash
 bunx oh-my-opencode install
+# or use npx if bunx doesn't work
+npx oh-my-opencode install
 ```
 
+> **Note for Ubuntu/Debian users**: If you installed Bun via Snap (`/snap/bin/bun`), `bunx` will fail with "script not found" due to Snap's sandboxing. Either use `npx` instead, or reinstall Bun via the official installer: `curl -fsSL https://bun.sh/install | bash`
+
 Follow the prompts to configure your Claude, ChatGPT, and Gemini subscriptions. After installation, authenticate your providers as instructed.
 
 **Alternative: Let an LLM Agent do it**