feat: 配置 golangci-lint 静态分析并修复存量违规

- 新增 backend/.golangci.yml 配置 12 个 linter（forbidigo、errorlint、errcheck、staticcheck、revive、gocritic、gosec、bodyclose、noctx、nilerr、goimports、gocyclo）
- 新增 lefthook.yml 配置 pre-commit hook 自动运行 lint
- 修复存量代码违规：errors.Is/As 替换、zap.Error 替换、import 排序、errcheck 修复
- 更新 README 补充编码规范说明
- 归档 backend-code-lint 变更

backend/internal/conversion/openai/stream_encoder.go

@@ -10,9 +10,9 @@ import (
 
 // StreamEncoder OpenAI 流式编码器
 type StreamEncoder struct {
-	bufferedStart      *canonical.CanonicalStreamEvent
-	toolCallIndexMap   map[string]int
-	nextToolCallIndex  int
+	bufferedStart     *canonical.CanonicalStreamEvent
+	toolCallIndexMap  map[string]int
+	nextToolCallIndex int
 }
 
 // NewStreamEncoder 创建 OpenAI 流式编码器
@@ -195,8 +195,8 @@ func (e *StreamEncoder) encodeMessageDelta(event canonical.CanonicalStreamEvent)
 func (e *StreamEncoder) encodeDelta(delta map[string]any) [][]byte {
 	chunk := map[string]any{
 		"choices": []map[string]any{{
-			"index":         0,
-			"delta":         delta,
+			"index": 0,
+			"delta": delta,
 		}},
 	}
 	return e.marshalChunk(chunk)