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/engine_test.go

@@ -38,8 +38,8 @@ func newMockAdapter(name string, passthrough bool) *mockProtocolAdapter {
 	}
 }
 
-func (m *mockProtocolAdapter) ProtocolName() string  { return m.protocolName }
-func (m *mockProtocolAdapter) ProtocolVersion() string { return "1.0" }
+func (m *mockProtocolAdapter) ProtocolName() string      { return m.protocolName }
+func (m *mockProtocolAdapter) ProtocolVersion() string   { return "1.0" }
 func (m *mockProtocolAdapter) SupportsPassthrough() bool { return m.passthrough }
 
 func (m *mockProtocolAdapter) DetectInterfaceType(nativePath string) InterfaceType {
@@ -190,14 +190,16 @@ func (m *mockProtocolAdapter) RewriteResponseModelName(body []byte, newModel str
 // noopStreamDecoder 空流式解码器
 type noopStreamDecoder struct{}
 
-func (d *noopStreamDecoder) ProcessChunk(rawChunk []byte) []canonical.CanonicalStreamEvent { return nil }
-func (d *noopStreamDecoder) Flush() []canonical.CanonicalStreamEvent                       { return nil }
+func (d *noopStreamDecoder) ProcessChunk(rawChunk []byte) []canonical.CanonicalStreamEvent {
+	return nil
+}
+func (d *noopStreamDecoder) Flush() []canonical.CanonicalStreamEvent { return nil }
 
 // noopStreamEncoder 空流式编码器
 type noopStreamEncoder struct{}
 
 func (e *noopStreamEncoder) EncodeEvent(event canonical.CanonicalStreamEvent) [][]byte { return nil }
-func (e *noopStreamEncoder) Flush() [][]byte                                          { return nil }
+func (e *noopStreamEncoder) Flush() [][]byte                                           { return nil }
 
 // ============ 测试用例 ============
 
@@ -615,6 +617,7 @@ func (d *engineTestStreamDecoder) ProcessChunk(raw []byte) []canonical.Canonical
 	}
 	return nil
 }
+
 func (d *engineTestStreamDecoder) Flush() []canonical.CanonicalStreamEvent {
 	if d.flushFn != nil {
 		return d.flushFn()
@@ -634,6 +637,7 @@ func (e *engineTestStreamEncoder) EncodeEvent(event canonical.CanonicalStreamEve
 	}
 	return nil
 }
+
 func (e *engineTestStreamEncoder) Flush() [][]byte {
 	if e.flushFn != nil {
 		return e.flushFn()