feat: Agent技能市场 — 公开/发布/分类/精选/评分

- AgentCreate 新增 category/tags/is_public/is_featured 字段
- 迁移 011 添加 agents 市场相关列（category/tags/is_public/rating_avg 等）
- agent_market API：按分类/标签/精选筛选，排序，分页
- 前端 AgentMarket.vue：市场浏览页，搜索/筛选/安装

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/agents.py

@@ -37,6 +37,10 @@ class AgentCreate(BaseModel):
     description: Optional[str] = None
     workflow_config: Dict[str, Any]  # 包含nodes和edges
     budget_config: Optional[Dict[str, Any]] = None
+    category: Optional[str] = None
+    tags: Optional[List[str]] = None
+    is_public: bool = False
+    is_featured: bool = False
 
 
 class AgentUpdate(BaseModel):
@@ -218,7 +222,11 @@ async def create_agent(
         workflow_config=agent_data.workflow_config,
         budget_config=agent_data.budget_config,
         user_id=current_user.id,
-        status="draft"
+        status="draft",
+        category=agent_data.category,
+        tags=agent_data.tags,
+        is_public=1 if agent_data.is_public else 0,
+        is_featured=1 if agent_data.is_featured else 0,
     )
     db.add(agent)
     db.commit()