add opencode runtime adapter and matrix fallback orchestration guidance

bin/mosaic-link-runtime-assets

@@ -80,5 +80,11 @@ for runtime_file in \
   copy_file_managed "$src" "$HOME/.claude/$runtime_file"
 done
 
+# OpenCode runtime adapter (optional, copied when present).
+opencode_adapter="$MOSAIC_HOME/runtime/opencode/AGENTS.md"
+if [[ -f "$opencode_adapter" ]]; then
+  copy_file_managed "$opencode_adapter" "$HOME/.config/opencode/AGENTS.md"
+fi
+
 echo "[mosaic-link] Runtime assets synced (non-symlink mode)"
 echo "[mosaic-link] Canonical source: $MOSAIC_HOME"