Add reference design images, point task.json at relative paths

tasks/c15/task.json

@@ -14,7 +14,7 @@
  ],
  "domain": "电商",
  "seed_meta": {
-  "ref_image_src": null
+  "ref_image_src": "prototypes/reference.jpg"
  },
  "title": "Simply Noted 手写信营销站：复刻、版式重构、区块精简与模板选配交互",
  "milestones": [