fix: chat stream closes when stripMarkers produces empty output

When a streaming chunk contains only markers/fences, stripMarkers
returns empty bytes. The 9P server interprets a zero-length Rread as
EOF, closing the client's FD.

Fix: loop in streamAgentChatText, advancing the offset past
marker-only chunks until real content arrives.

fs/handlers_agent.go

@@ -125,11 +125,18 @@ func streamAgentChat(ctx HandlerCtx, cctx context.Context, base string) ([]byte,
 func streamAgentChatText(ctx HandlerCtx, cctx context.Context, base string) ([]byte, string, error) {
 	merged, cancel := mergeCtx(ctx, cctx)
 	defer cancel()
-	data, nextBase, err := streamChat(ctx.AgentLog, merged, base)
-	if err != nil || len(data) == 0 {
-		return data, nextBase, err
+	for {
+		data, nextBase, err := streamChat(ctx.AgentLog, merged, base)
+		if err != nil || len(data) == 0 {
+			return data, nextBase, err
+		}
+		stripped := stripMarkers(data)
+		if len(stripped) > 0 {
+			return stripped, nextBase, nil
+		}
+		// All data was markers/fences — advance base and wait for more.
+		base = nextBase
 	}
-	return stripMarkers(data), nextBase, nil
 }
 
 // stripMarkers removes [[[...]]] marker lines and source fence lines from raw