feat(FN-3869): add github tracking fields and resolver to tasks

Adds GitHub tracking fields to the task model with schema migration, store accessors, and type definitions in `@fusion/core`. New `github-tracking.ts` module exposes a resolver for fetching GitHub issue/PR metadata, integrated into the task store. Test coverage spans the new module, store integratio

Fusion-Task-Id: FN-3869

plugins/fusion-plugin-roadmap/src/store/__tests__/roadmap-store.test.ts

@@ -744,7 +744,7 @@ describe("RoadmapStore", () => {
 
   describe("schema version", () => {
     it("schema version is 40 after init", () => {
-      expect(db.getSchemaVersion()).toBe(70);
+      expect(db.getSchemaVersion()).toBe(71);
     });
   });