fix: filter empty assistant messages in _save_turn instead of patching at send time

nanobot/agent/loop.py

@@ -451,6 +451,8 @@ class AgentLoop:
         for m in messages[skip:]:
             entry = {k: v for k, v in m.items() if k != "reasoning_content"}
             role, content = entry.get("role"), entry.get("content")
+            if role == "assistant" and not content and not entry.get("tool_calls"):
+                continue  # skip empty assistant messages — they poison session context
             if role == "tool" and isinstance(content, str) and len(content) > self._TOOL_RESULT_MAX_CHARS:
                 entry["content"] = content[:self._TOOL_RESULT_MAX_CHARS] + "\n... (truncated)"
             elif role == "user":