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

backend/tests/unit/test_settings_supabase_env.py

@@ -23,27 +23,13 @@ def test_social_prefixed_supabase_env_populates_settings(
     settings = Settings()
 
     assert settings.supabase.public_url == "https://public.example:8443"
-    assert settings.supabase.api_external_url == "https://public.example:8443"
     assert settings.supabase.anon_key == "anon-key"
     assert settings.supabase.service_role_key == "service-key"
     assert settings.supabase.jwt_secret == "jwt-secret"
 
     supabase_settings = settings.model_dump()["supabase"]
     assert supabase_settings["public_url"] == "https://public.example:8443"
-    assert supabase_settings["api_external_url"] == "https://public.example:8443"
     assert supabase_settings["anon_key"] == "anon-key"
     assert supabase_settings["service_role_key"] == "service-key"
     assert supabase_settings["jwt_secret"] == "jwt-secret"
     assert settings.database_url == "postgresql+asyncpg://user:pass@db:5432/app"
-
-
-def test_social_prefixed_api_external_url_is_loaded(
-    monkeypatch: MonkeyPatch,
-) -> None:
-    monkeypatch.setenv("SOCIAL_SUPABASE__PUBLIC_SCHEME", "https")
-    monkeypatch.setenv("SOCIAL_SUPABASE__PUBLIC_HOST", "api.example")
-    monkeypatch.setenv("SOCIAL_SUPABASE__KONG_HTTP_PORT", "8443")
-
-    settings = Settings()
-
-    assert settings.supabase.api_external_url == "https://api.example:8443"