refactor: 删除未使用的 api_external_url 配置并完善 runtime 文档

- 删除 SupabaseSettings 中未使用的 api_external_url computed field
- 更新测试文件移除相关测试用例
- backend/AGENTS.md 新增软删除设计规则
- runtime-database.md 更新表结构（删除 user_agents，表名更新为 agent_chat_sessions/messages，system_agents）
- runtime-frontend.md 补充路由结构和功能模块说明
- 根 AGENTS.md 清理过时技能路径引用

AGENTS.md

@@ -17,7 +17,6 @@ social-app/
 Follow this hierarchy when developing:
 
 ```
-~/.config/opencode/AGENTS.md          # Global core rules (skills, agents, process)
 ├── This file (root AGENTS.md)        # Project-level entry
 │   ├── backend/AGENTS.md              # Backend-specific rules
 │   └── apps/AGENTS.md                 # Frontend-specific rules
@@ -41,8 +40,6 @@ Follow this hierarchy when developing:
 
 ## Skills (Domain Knowledge)
 
-Project-specific skills are available in `.opencode/skills/`:
-
 | Skill | Purpose | When to Use |
 |-------|---------|-------------|
 | **ag-ui** | AG-UI protocol for agent-user interaction | Agent chat, streaming events, tool calls, state sync |