test(core): temporary diagnostic for failing agent S2 writes

Logs the S2 append pipeline error (basin, stream, endpoint, message) when
a session .out write fails, to capture why the agent's direct-to-S2 writes
return an empty stream in CI while succeeding locally. To be reverted.

Author: ericallam

packages/core/src/v3/realtimeStreams/streamsWriterV2.ts

@@ -193,6 +193,17 @@ export class StreamsWriterV2<T = any> implements StreamsWriter {
         this.log("[S2MetadataStream] Stream error occurred but stream was aborted");
         return;
       }
+      console.error(
+        "[S2WRITER-DIAG] pipeline error basin=",
+        this.options.basin,
+        "stream=",
+        this.options.stream,
+        "endpoint=",
+        this.options.endpoint,
+        "err=",
+        error instanceof Error ? `${error.name}: ${error.message}` : String(error),
+        (error instanceof Error ? error.stack ?? "" : "").split("\n").slice(1, 5).join(" || ")
+      );
       this.logError("[S2MetadataStream] Error in stream pipeline:", error);
       throw error;
     }