feat: add kanban board

src/kanban/outboundMessages.ts

@@ -0,0 +1,7 @@
+import { AppError } from "@/error";
+import { KanbanBoard } from "./buildKanbanBoard";
+
+/** Host → webview. */
+export type KanbanOutboundMessage =
+	| { type: "state"; board: KanbanBoard }
+	| { type: "error"; error: AppError };