feat: LLM可自主操作提醒 — reminder_create/list/delete 工具

- 新增 GatewayClient HTTP 客户端，调用 Gateway /api/v1/reminders API
- 新增 3 个提醒工具：reminder_create / reminder_list / reminder_delete
- main.go 注册提醒工具，需 GATEWAY_URL + INTERNAL_SERVICE_TOKEN 环境变量
- AutonomousToolPolicy 白名单加入提醒工具，允许自主思考中创建提醒

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

backend/ai-core/internal/background/thinker.go

@@ -211,6 +211,7 @@ func DefaultAutonomousToolPolicy() *AutonomousToolPolicy {
 			"calculator", "datetime", "web_fetch",
 			"host_exec", "host_file", "host_system",
 			"vision_analyze", "knowledge_search", "knowledge_ingest",
+			"reminder_create", "reminder_list", "reminder_delete",
 		},
 		MaxToolCallsPerRound: 5,
 		MaxHighRiskPerHour:   10,