清理城市农业类别 + 修复采集器LLM直接选题

清理:
- sustainability_cases.json移除GLO-001/CHN-001城市农业案例,替换为循环消费
- initial_cases.json移除case8东京垂直农场/case26城市屋顶农场
- strategy_topics_to_json.py移除B01/B05/D05三个种菜选题
- collector.py移除城市农业→循环消费映射,更新注释
- 删除fix_collector.py/test_image_gen.py/generate_images.py等遗留脚本
- 删除import_topics.py和automation/下旧版生成脚本

修复:
- collector.py _generate_topic_with_llm不再依赖搜索结果,无搜索时LLM直接生成
- run()始终调用LLM,不再要求web_search_results非空
- 替换sources.yaml中已失效的RSS源(澎湃/虎嗅/中新网→36氪/少数派)

automation/data/initial_cases.json

@@ -83,18 +83,6 @@
     "credibility_rating": "⭐⭐⭐⭐",
     "china_applicability": "⭐⭐⭐⭐"
   },
-  {
-    "id": 8,
-    "title": "东京垂直农场",
-    "field": "可持续生活系统",
-    "summary": "利用高层建筑内部空间进行多层种植，实现都市粮食自给。",
-    "key_metrics": "单位面积产量是传统农业的10倍，节水90%",
-    "date": "2024",
-    "source": "Spread 公司",
-    "source_url": "https://www.spread.co.jp/",
-    "credibility_rating": "⭐⭐⭐⭐⭐",
-    "china_applicability": "⭐⭐⭐"
-  },
   {
     "id": 9,
     "title": "纽约社区花园政策",
@@ -299,18 +287,6 @@
     "credibility_rating": "⭐⭐⭐",
     "china_applicability": "⭐⭐⭐⭐⭐"
   },
-  {
-    "id": 26,
-    "title": "城市屋顶农场：上海社区的粮食自给实验",
-    "field": "可持续生活系统",
-    "summary": "在上海某小区屋顶建设 200㎡ 农场，一年内生产 600kg 蔬菜，减少碳足迹 1.2 吨。",
-    "key_metrics": "蔬菜自给率 40%, 参与家庭 50 户, 社区互动提升 300%",
-    "date": "2024",
-    "source": "城市农业网",
-    "source_url": "https://www.urbanfarming.org/",
-    "credibility_rating": "⭐⭐⭐⭐",
-    "china_applicability": "⭐⭐⭐⭐⭐"
-  },
   {
     "id": 27,
     "title": "数字游民签证地图：2026 最新政策对比",