refactor: use nginx -s reload instead of systemd

- doc: add comment

internal/systemd/systemctl.go

@@ -12,19 +12,25 @@ import (
 )
 
 const (
-	loadStateProperty   = "LoadState"
+	// loadStateProperty systemctl show 输出的加载状态属性名。
+	loadStateProperty = "LoadState"
+	// activeStateProperty systemctl show 输出的活动状态属性名。
 	activeStateProperty = "ActiveState"
-	subStateProperty    = "SubState"
-	loadedState         = "loaded"
-	notFoundState       = "not-found"
+	// subStateProperty systemctl show 输出的子状态属性名。
+	subStateProperty = "SubState"
+	// loadedState 表示单元已加载。
+	loadedState = "loaded"
+	// notFoundState 表示单元未找到。
+	notFoundState = "not-found"
 )
 
-// Systemctl invokes one exact systemctl executable directly, never through a shell.
+// Systemctl 直接调用一个精确的 systemctl 可执行文件，绝不经过 shell。
 type Systemctl struct {
+	// executable systemctl 的绝对路径。
 	executable string
 }
 
-// NewSystemctl requires the absolute executable path supplied by local daemon configuration.
+// NewSystemctl 要求传入本地守护进程配置提供的可执行文件绝对路径。
 func NewSystemctl(executable string) (*Systemctl, error) {
 	if !filepath.IsAbs(executable) {
 		return nil, errors.New("systemctl executable path must be absolute")
@@ -32,6 +38,8 @@ func NewSystemctl(executable string) (*Systemctl, error) {
 	return &Systemctl{executable: executable}, nil
 }
 
+// Inspect 查询指定单元的关键状态属性并返回其状态。
+// 未找到单元时错误包装 ErrUnitNotFound；加载状态异常时返回错误。
 func (s *Systemctl) Inspect(ctx context.Context, unitName string) (Unit, error) {
 	if err := validateUnitName(unitName); err != nil {
 		return Unit{}, err
@@ -62,14 +70,17 @@ func (s *Systemctl) Inspect(ctx context.Context, unitName string) (Unit, error)
 	return unit, nil
 }
 
+// Start 启动指定单元。
 func (s *Systemctl) Start(ctx context.Context, unitName string) error {
 	return s.changeState(ctx, "start", unitName)
 }
 
+// Stop 停止指定单元。
 func (s *Systemctl) Stop(ctx context.Context, unitName string) error {
 	return s.changeState(ctx, "stop", unitName)
 }
 
+// changeState 通过 systemctl 执行给定动作（start 或 stop）。
 func (s *Systemctl) changeState(ctx context.Context, action, unitName string) error {
 	if err := validateUnitName(unitName); err != nil {
 		return err
@@ -81,6 +92,8 @@ func (s *Systemctl) changeState(ctx context.Context, action, unitName string) er
 	return nil
 }
 
+// parseUnitProperties 解析 systemctl show 的输出，
+// 严格要求恰好包含三个预期属性且不重复、不多余。
 func parseUnitProperties(unitName string, output []byte) (Unit, error) {
 	values := make(map[string]string, 3)
 	scanner := bufio.NewScanner(bytes.NewReader(output))
@@ -116,6 +129,7 @@ func parseUnitProperties(unitName string, output []byte) (Unit, error) {
 	}, nil
 }
 
+// validateUnitName 校验单元名称必须非空且不含首尾空白。
 func validateUnitName(unitName string) error {
 	if unitName == "" || strings.TrimSpace(unitName) != unitName {
 		return errors.New("exact systemd unit name is required")
@@ -123,6 +137,7 @@ func validateUnitName(unitName string) error {
 	return nil
 }
 
+// commandError 包装命令执行错误，并附带去除首尾空白的命令输出作为诊断细节。
 func commandError(action string, output []byte, err error) error {
 	detail := strings.TrimSpace(string(output))
 	if detail == "" {