p9: remove newline event handling; reset assistantStarted on user event

Drop formatEvent "newline" case (event no longer emitted). Reset
assistantStarted on "user" event instead so chained turns get the
"assistant: " prefix correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/p9/server.go

@@ -1354,8 +1354,6 @@ func formatEvent(ev agent.Event) []byte {
 		return []byte("agent stalled\n")
 	case "info":
 		return []byte(ev.Content)
-	case "newline":
-		return []byte("\n")
 	default:
 		return nil
 	}

internal/p9/sessionfilestore.go

@@ -325,7 +325,7 @@ func (s *SessionFileStore) makePublish() func(agent.Event) {
 	assistantStarted := false
 	return func(ev agent.Event) {
 		switch ev.Role {
-		case "call", "tool", "newline":
+		case "user", "call", "tool":
 			assistantStarted = false
 		case "assistant":
 			if !assistantStarted {