Assistant house style: a drawing-convention text every AI assistant reads

QetSettings::houseStyle()/setHouseStyle() (assistant/house_style),
qet.houseStyle()/qet.setHouseStyle() in the script API, a house_style
field in qet-assistant.json, and qet_about in the qet MCP server
showing it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/utils/qetsettings.h

@@ -41,6 +41,16 @@ namespace QetSettings
 	bool liveAssistantEnabled();
 	void setLiveAssistantEnabled(bool enabled);
 
+	/**
+		@brief Free-text drawing conventions for this installation (grid,
+		flow direction, routing, tagging, grouping, ...), written once by
+		the user and read by every AI assistant that connects -- live or
+		headless -- through qet-assistant.json and qet.houseStyle().
+		Empty when never set.
+	*/
+	QString houseStyle();
+	void setHouseStyle(const QString &text);
+
 	/**
 		@brief The sheet (folio) background the diagram editors draw.