feat(proto): add StepOutputChunk streaming message

proto/vantage/v1/vantage.proto

@@ -56,6 +56,7 @@ message AgentMessage {
     AgentReady    ready  = 3;
     CommandResult result = 4;
     StepResult step_result = 5;
+    StepOutputChunk step_output = 6;
   }
 }
 
@@ -125,3 +126,10 @@ message StepResult {
   string stderr                 = 4;
   map<string, string> output_env = 5;
 }
+
+message StepOutputChunk {
+  string command_id = 1;
+  uint64 seq        = 2;
+  bytes  data       = 3;
+  bool   eof        = 4;
+}