fix(#1201): prevent sync_agents from dropping all agents on list() failure

hive-c0re/src/meta.rs

@@ -80,6 +80,21 @@ pub async fn sync_agents(
         return Ok(());
     }
 
+    // Safety guard: refuse to write an empty agent list over a non-empty
+    // on-disk flake. An empty `agents` slice is never intentional — it
+    // means `nixos-container list` failed and the caller got an empty
+    // fallback. Overwriting here would drop every agent from the meta
+    // flake and trigger unnecessary (and potentially destructive) cascade
+    // rebuilds. Callers that genuinely need to clear the agent list
+    // (there are none today) must handle this case explicitly.
+    if agents.is_empty() && !initial && !on_disk.is_empty() {
+        tracing::warn!(
+            "sync_agents: refusing to overwrite non-empty meta flake with empty agent list \
+             (nixos-container list may have failed)"
+        );
+        return Ok(());
+    }
+
     std::fs::write(&flake_path, &new_flake)
         .with_context(|| format!("write {}", flake_path.display()))?;