fix(steering): skip phantom zero-turn result on --resume; guard empty replies

On --resume, a background task stopped by the previous restart is replayed
as its own zero-turn turn with an empty result, before the real turn.
consume_stream took it as the answer -> empty Discord message (400) ->
"Sorry, something went wrong", while the real turn ran orphaned.

- stream_json: skip result with num_turns=0, empty, non-error
- router: empty Claude response becomes a visible notice, not an adapter crash

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014y1sJNe6mkWwFMwkWmCg8J

tests/test_router.py

@@ -195,6 +195,13 @@ class TestRegularMessage:
         assert is_cmd is False
         mock_send.assert_called_once_with("ch-1", "hello", model="sonnet", on_text=None, voice_mode=False, adapter_name=None)
 
+    @patch("src.router._get_channel_config", return_value=None)
+    @patch("src.router.send_message", return_value="  ")
+    def test_empty_response_becomes_notice(self, mock_send, _chan):
+        response, is_cmd = route_message("ch-1", "user-1", "hello", model="sonnet")
+        assert response.strip()
+        assert is_cmd is False
+
     @patch("src.router.send_message")
     def test_model_override(self, mock_send):
         mock_send.return_value = "Response"