List clusters visible to the gateway

Returns the cluster catalog visible to the gateway serving this request. The entry with `local: true` is the cluster that owns native-local resources (a sandbox created here without any `{clusterID}.` prefix lives in that cluster). Other entries are reachable via cross-cluster forwarding: prefix sandbox IDs with `{clusterID}.` (dot) and pool names with `{clusterID}::` (double colon) to route to them. This endpoint requires only a regular API key — it does not leak gateway URLs, headers, or any write-capability.

curl -X GET "https://clusters"

{  "clusters": [    {      "id": "cluster1",      "name": "cluster1",      "local": true    }  ]}

{  "error": "string",  "errorCode": "API_KEY_REQUIRED",  "detail": null}

{  "error": "string",  "errorCode": "API_KEY_REQUIRED",  "detail": null}