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/anthropic/stream_decoder.go

@@ -28,7 +28,7 @@ func NewStreamDecoder() *StreamDecoder {
 func (d *StreamDecoder) ProcessChunk(rawChunk []byte) []canonical.CanonicalStreamEvent {
 	data := rawChunk
 	if len(d.utf8Remainder) > 0 {
-		data = append(d.utf8Remainder, rawChunk...)
+		data = append(append([]byte{}, d.utf8Remainder...), rawChunk...)
 		d.utf8Remainder = nil
 	}
 
@@ -50,9 +50,10 @@ func (d *StreamDecoder) ProcessChunk(rawChunk []byte) []canonical.CanonicalStrea
 
 	for _, line := range strings.Split(text, "\n") {
 		line = strings.TrimRight(line, "\r")
-		if strings.HasPrefix(line, "event: ") {
+		switch {
+		case strings.HasPrefix(line, "event: "):
 			eventType = strings.TrimPrefix(line, "event: ")
-		} else if strings.HasPrefix(line, "data: ") {
+		case strings.HasPrefix(line, "data: "):
 			eventData = strings.TrimPrefix(line, "data: ")
 			if eventType != "" && eventData != "" {
 				chunkEvents := d.processEvent(eventType, []byte(eventData))
@@ -60,8 +61,8 @@ func (d *StreamDecoder) ProcessChunk(rawChunk []byte) []canonical.CanonicalStrea
 			}
 			eventType = ""
 			eventData = ""
-		} else if line == "" {
-			// SSE 事件分隔符
+		case line == "":
+			continue
 		}
 	}
 
@@ -135,7 +136,7 @@ func (d *StreamDecoder) processMessageStart(data []byte) []canonical.CanonicalSt
 // processContentBlockStart 处理内容块开始事件
 func (d *StreamDecoder) processContentBlockStart(data []byte) []canonical.CanonicalStreamEvent {
 	var raw struct {
-		Index        int    `json:"index"`
+		Index        int `json:"index"`
 		ContentBlock struct {
 			Type     string `json:"type"`
 			Text     string `json:"text"`