Replace vim-tiny with neovim from GitHub releases (pinned, multi-arch).
Add bat, eza, zoxide from GitHub releases and tmux, htop, patch from apt.
Move tmux from OMOS-only to base image. Set EDITOR=nvim.
Add neovim config mount option to docker-compose and docs.
Integrate oh-my-opencode-slim as an opt-in feature via INSTALL_OMOS build arg.
A single build arg installs Bun, tmux, and the plugin; runtime activation is
controlled by ENABLE_OMOS and related env vars in the entrypoint.