refactor: use unused constants instead of removing them

- Export unitTypes as UnitTypes and add HasValidUnitSuffix helper
- Use killed const (exit code 130) in execute() to detect SIGINT
- Update go.mod to go 1.26

util.go

@@ -11,6 +11,9 @@ import (
 
 var systemctl string
 
+// killed is the exit code returned when a process is terminated by SIGINT.
+const killed = 130
+
 func init() {
 	path, _ := exec.LookPath("systemctl")
 	systemctl = path
@@ -37,6 +40,10 @@ func execute(ctx context.Context, args []string) (string, string, int, error) {
 	warnings = stderr.String()
 	code = cmd.ProcessState.ExitCode()
 
+	if code == killed {
+		return output, warnings, code, ErrExecTimeout
+	}
+
 	customErr := filterErr(warnings)
 	if customErr != nil {
 		err = customErr