fix: 移除早间简报功能 + 修复多处"人机感"用语

- 删除 briefing_handler.go / briefing_store.go 及所有相关路由与配置
- 移除 Gateway Config 中 ToolEngineURL / BriefingTime 字段
- 移除 DevTools 中 gateway 的 TOOL_ENGINE_URL 环境变量
- webhook 错误提示从"AI 服务异常/暂不可用"改为昔涟自然口吻
- markdown 导出中昔涟头像从 🤖 改为 💫
- 后台思考提示词"系统会误解析"改为"我会误解析"

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/gateway/internal/handler/webhook_handler.go

@@ -96,7 +96,7 @@ func (h *WebhookHandler) HandleGenericWebhook(c *gin.Context) {
 	resp, err := h.callAICore(userID, sessionID, req.Message, mode, platform)
 	if err != nil {
 		logger.Printf("[webhook] AI-Core 调用失败 (platform=%s): %v", platform, err)
-		c.JSON(502, GenericWebhookResponse{Error: "AI 服务暂不可用: " + err.Error()})
+		c.JSON(502, GenericWebhookResponse{Error: "昔涟暂时无法回应喵...（请稍后再试）"})
 		return
 	}
 
@@ -247,7 +247,7 @@ func (h *WebhookHandler) HandleDiscordWebhook(c *gin.Context) {
 		logger.Printf("[webhook:discord] AI-Core 调用失败: %v", err)
 		c.JSON(200, DiscordResponse{
 			Type: 4,
-			Data: &DiscordResponseData{Content: "昔涟暂时无法回应喵...（AI 服务异常: " + err.Error() + "）"},
+			Data: &DiscordResponseData{Content: "昔涟暂时无法回应喵...（遇到了一些问题，请稍后再试）"},
 		})
 		return
 	}