fix(#1897): treat empty HYPERHIVE_HIVE_DOMAIN as unset in HiveDomain

Per argus review: std::env::var(..).ok() yields Some("") for an empty
env value, so require_hive_domain would 'succeed' with an empty domain
and emit invalid nix (swarm.peers."" = …). Filter empty so it resolves
to None → the CLI's clear 'domain unset' error fires instead.

hive-c0re/src/server.rs

@@ -117,9 +117,13 @@ async fn dispatch(req: &HostRequest, coord: Arc<Coordinator>) -> HostResponse {
             // The hive domain is injected into c0re's service env by
             // hive-c0re.nix (`HYPERHIVE_HIVE_DOMAIN`); surface it so the
             // operator CLI can fill in this hive's own identity.
-            HostRequest::HiveDomain => {
-                HostResponse::hive_domain(std::env::var("HYPERHIVE_HIVE_DOMAIN").ok())
-            }
+            HostRequest::HiveDomain => HostResponse::hive_domain(
+                // Treat an empty env value as unset — otherwise the CLI
+                // would emit `swarm.peers."" = …`, invalid nix.
+                std::env::var("HYPERHIVE_HIVE_DOMAIN")
+                    .ok()
+                    .filter(|d| !d.is_empty()),
+            ),
             HostRequest::Pending => HostResponse::pending(coord.approvals.pending()?),
             HostRequest::Approve { id } => {
                 actions::approve(coord.clone(), *id).await?;