refactor: use nginx -s reload instead of systemd

- doc: add comment

internal/systemd/systemctl_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 )
 
+// TestParseUnitPropertiesRequiresExactProperties 验证属性解析严格接受恰好三个预期属性，并拒绝缺失、多余或重复属性。
 func TestParseUnitPropertiesRequiresExactProperties(t *testing.T) {
 	t.Parallel()
 	unit, err := parseUnitProperties("yms-green.service", []byte("LoadState=loaded\nActiveState=inactive\nSubState=dead\n"))
@@ -26,6 +27,7 @@ func TestParseUnitPropertiesRequiresExactProperties(t *testing.T) {
 	}
 }
 
+// TestNewSystemctlRequiresAbsoluteExecutable 验证相对路径会被拒绝，绝对路径会被接受。
 func TestNewSystemctlRequiresAbsoluteExecutable(t *testing.T) {
 	t.Parallel()
 	if _, err := NewSystemctl("systemctl"); err == nil {
@@ -37,6 +39,7 @@ func TestNewSystemctlRequiresAbsoluteExecutable(t *testing.T) {
 	}
 }
 
+// TestValidateUnitNamePreservesOpaqueValue 验证合法单元名被原样保留，空串或带首尾空白被拒绝。
 func TestValidateUnitNamePreservesOpaqueValue(t *testing.T) {
 	t.Parallel()
 	if err := validateUnitName("backend-green.service"); err != nil {