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_claude_runner.py

@@ -349,3 +349,8 @@ def test_max_live_non_positive_logs_and_always_degrades(caplog):
         registry = claude_runner.RunnerRegistry(max_live=0, idle_minutes=20)
     assert any("max_live" in rec.message for rec in caplog.records)
     assert registry.get("any-channel") is None
+
+
+def test_stream_json_self_check_skips_phantom_resume_result():
+    from src import stream_json
+    stream_json.demo()