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/encoder.go

@@ -182,7 +182,7 @@ func encodeContentBlocks(blocks []canonical.ContentBlock) []map[string]any {
 			result = append(result, m)
 		case "tool_result":
 			m := map[string]any{
-				"type":       "tool_result",
+				"type":        "tool_result",
 				"tool_use_id": b.ToolUseID,
 			}
 			if b.Content != nil {
@@ -335,11 +335,11 @@ func encodeResponse(resp *canonical.CanonicalResponse) ([]byte, error) {
 	}
 
 	result := map[string]any{
-		"id":      resp.ID,
-		"type":    "message",
-		"role":    "assistant",
-		"model":   resp.Model,
-		"content": blocks,
+		"id":            resp.ID,
+		"type":          "message",
+		"role":          "assistant",
+		"model":         resp.Model,
+		"content":       blocks,
 		"stop_reason":   sr,
 		"stop_sequence": nil,
 		"usage":         usage,