## Summary

- Add an ACP-mode guard to the documented OpenCode bell plugin snippet
- Preserve terminal bell notifications for Terminal Threads while
avoiding writes to stdout when OpenCode is used as an ACP External Agent

## Rationale

The OpenCode bell plugin writes BEL to stdout for Terminal Thread
notifications. When the plugin is installed globally and OpenCode runs
in ACP mode, stdout is the JSON-RPC transport. OpenCode sets
`OPENCODE_CLIENT=acp` in this mode, so the guard prevents BEL bytes from
corrupting ACP JSON-RPC messages, including usage updates and permission
flows.

Release Notes:

- N/A