chore(lint): unify formatting on golangci-lint fmt (gofumpt), uncap issue reporting, fix gofumpt drift

- make format 现在与 code-check 使用同一格式化器（golangci-lint fmt），消除 goimports -local 与 gofumpt 的格式拉锯
- .golangci.yml 关闭默认 50/3 截断，完整上报所有问题（只增强不弱化）
- 全库 gofumpt 规范化（203 files, 纯格式无行为变更）

backend/downstream/plugins/custom_example/plugin.go

@@ -6,12 +6,11 @@
 package custom_example
 
 import (
+	"Wavelet/core"
+	"Wavelet/core/contracts"
 	"net/http"
 
 	"github.com/gin-gonic/gin"
-
-	"Wavelet/core"
-	"Wavelet/core/contracts"
 )
 
 // Plugin implements core.Plugin for the custom_example downstream plugin.