## Why
Core tests depend on `codex-app-server` to install and clean up agent message boards. A dedicated host crate lets them use this functionality without depending on the app server.
## What changed
Move the existing host implementation into `codex-agent-message-board-host` and update core test helpers and scenarios to use it directly. Keep the installation and deletion functions re-exported from `codex-app-server`.
GitOrigin-RevId: 9ed62d50a5f31c70550d70ba3c583399e869168a
