Use Bun baseline build for AVX2-less CPU compatibility (Sandy Bridge)

Dockerfile

@@ -163,10 +163,23 @@ RUN if [ "${INSTALL_GO}" = "true" ]; then \
 # ── Optional: oh-my-opencode-slim (multi-agent orchestration) ────────
 # Installs Bun runtime and the oh-my-opencode-slim npm package.
 # Runtime activation is controlled by ENABLE_OMOS env var in entrypoint.
+# Uses the baseline Bun build (SSE4.2 only) for compatibility with older
+# CPUs that lack AVX2 (e.g. Sandy Bridge on OpenStack).
 ARG INSTALL_OMOS=false
 ARG OMOS_VERSION=latest
 RUN if [ "${INSTALL_OMOS}" = "true" ]; then \
-      curl -fsSL https://bun.sh/install | BUN_INSTALL=/usr/local bash && \
+      ARCH=$(uname -m) && \
+      if [ "$ARCH" = "x86_64" ]; then \
+        BUN_ARCH="x64-baseline"; \
+      elif [ "$ARCH" = "aarch64" ]; then \
+        BUN_ARCH="aarch64"; \
+      fi && \
+      BUN_VERSION=$(curl -fsSL https://github.com/oven-sh/bun/releases/latest/download/LATEST) && \
+      curl -fsSL "https://github.com/oven-sh/bun/releases/latest/download/bun-linux-${BUN_ARCH}.zip" -o /tmp/bun.zip && \
+      unzip -o /tmp/bun.zip -d /tmp/bun && \
+      mv /tmp/bun/bun-linux-${BUN_ARCH}/bun /usr/local/bin/bun && \
+      chmod +x /usr/local/bin/bun && \
+      rm -rf /tmp/bun /tmp/bun.zip && \
       bun --version && \
       npm install -g oh-my-opencode-slim@${OMOS_VERSION}; \
     fi