Fix TypeError when writing report sections that contain lists

The `save_report_section_decorator` calls `f.write(content)` but some
report sections (e.g. `trader_investment_plan`) can be a list instead
of a string, causing `TypeError: write() argument must be str, not list`.

Handle this by joining list items with newlines before writing.

cli/main.py

@@ -973,7 +973,10 @@ def run_analysis():
                 if content:
                     file_name = f"{section_name}.md"
                     with open(report_dir / file_name, "w", encoding="utf-8") as f:
-                        f.write(content)
+                        if isinstance(content, list):
+                            f.write("\n".join(str(item) for item in content))
+                        else:
+                            f.write(content)
         return wrapper
 
     message_buffer.add_message = save_message_decorator(message_buffer, "add_message")