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.

Dockerfile

@@ -106,6 +106,19 @@ RUN if [ "${INSTALL_GO}" = "true" ]; then \
       ln -s /usr/local/go/bin/gofmt /usr/local/bin/gofmt; \
     fi
 
+# ── Optional: oh-my-opencode-slim (multi-agent orchestration) ────────
+# Installs Bun runtime, tmux, and the oh-my-opencode-slim npm package.
+# Runtime activation is controlled by ENABLE_OMOS env var in entrypoint.
+ARG INSTALL_OMOS=false
+ARG OMOS_VERSION=latest
+RUN if [ "${INSTALL_OMOS}" = "true" ]; then \
+      apt-get update && apt-get install -y --no-install-recommends tmux && \
+      rm -rf /var/lib/apt/lists/* && \
+      curl -fsSL https://bun.sh/install | BUN_INSTALL=/usr/local bash && \
+      bun --version && \
+      npm install -g oh-my-opencode-slim@${OMOS_VERSION}; \
+    fi
+
 # ── Non-root user ────────────────────────────────────────────────────
 ARG USER_NAME=developer
 ARG USER_UID=1000