fix: allow recipe with fewer required_nodes to run on larger cluster

When --cluster is specified, only gate on too-few nodes (actual < required)
instead of requiring an exact match.  auto-selection keeps exact match
unchanged.

src/docker/mod.rs

@@ -62,9 +62,9 @@ pub async fn serve(
     };
     if let Some(required) = recipe.required_nodes {
         let actual = cluster.nodes.len() as u32;
-        if actual != required {
+        if actual < required {
             anyhow::bail!(
-                "Recipe '{}' requires {} nodes, but cluster '{}' has {} nodes",
+                "Recipe '{}' requires {} nodes, but cluster '{}' has only {} nodes",
                 recipe.name,
                 required,
                 cluster.name,