feat(FN-3159): move roadmap ownership from core to fusion-plugin-roadmap pl

Moves roadmap ownership from `@fusion/core` to `fusion-plugin-roadmap`, deleting ~1,800 lines of roadmap store, types, ordering, and handoff logic from core and replacing it with ~760 lines of new route handlers in the plugin. The dashboard's roadmap routes and suggestions modules are substantially

Fusion-Task-Id: FN-3159

plugins/fusion-plugin-roadmap/src/index.ts

@@ -1,6 +1,6 @@
 import type { Database } from "@fusion/core";
 import { definePlugin } from "@fusion/plugin-sdk";
-import { createRoadmapPluginRoutes } from "./roadmap-routes.js";
+import { createRoadmapPluginRoutes } from "./routes/roadmap-routes.js";
 
 export function ensureRoadmapSchema(db: Database): void {
   db.exec(`