Guard document style against persona guessing

src/settings.py

@@ -136,6 +136,10 @@ DEFAULT_SETTINGS = {
     "task_model": "",
     "default_endpoint_id": "",
     "default_model": "",
+    # Optional prose style used only for normal document writing/editing.
+    # Email replies use email_writing_style instead because greetings,
+    # signatures, and mailbox identity rules are medium-specific.
+    "document_writing_style": "",
     # Ordered fallback chain for the default chat model. Each entry is
     # {"endpoint_id": "...", "model": "..."}. If the primary model fails
     # before producing output (endpoint offline / errors), the chat