Add optional oh-my-opencode-slim multi-agent orchestration support

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.

docker-compose.yml

@@ -15,6 +15,7 @@ services:
       args:
         INSTALL_PYTHON: "false"
         INSTALL_GO: "false"
+        INSTALL_OMOS: "false"
     image: opencode-devbox:latest
     container_name: opencode-devbox
     stdin_open: true
@@ -37,6 +38,9 @@ services:
       # - ~/.config/opencode/skills:/home/developer/.config/opencode/skills:ro
       # - ~/.agents/skills:/home/developer/.agents/skills:ro
 
+      # Optional: mount your own oh-my-opencode-slim config
+      # - ./oh-my-opencode-slim.json:/home/developer/.config/opencode/oh-my-opencode-slim.json:ro
+
       # Optional: persist opencode data (auth, memory, etc.)
       - devbox-data:/home/developer/.local/share/opencode