Hold a unit an administrator installed whatever its state, and a packaged unit only when the machine uses it (hq ADR 0103)

internal/system/arch.go

@@ -247,6 +247,22 @@ func (arch) AddUserToGroup(ctx context.Context, run Runner, name, group string)
 	return nil
 }
 
+// ServiceUnitFile says where the service manager loads a unit from — systemd's FragmentPath. It
+// is how the host tells a unit an administrator installed, under /etc or /run, from one a package
+// ships under /usr (novox/hq ADR 0103). Empty, with no error, for a unit that loads from nowhere.
+func (arch) ServiceUnitFile(ctx context.Context, run Runner, unit string) (string, error) {
+	out, err := run(ctx, "systemctl", "show", unit, "--property=FragmentPath")
+	if err != nil {
+		return "", fmt.Errorf("the service manager did not say where %s comes from: %w", unit, err)
+	}
+	for _, line := range strings.Split(out, "\n") {
+		if path, ok := strings.CutPrefix(strings.TrimSpace(line), "FragmentPath="); ok {
+			return strings.TrimSpace(path), nil
+		}
+	}
+	return "", nil
+}
+
 // ReloadUnits has systemd read its unit files again. A unit file that changed on disk is otherwise
 // ignored: a restart runs the unit systemd already loaded, and the new text only takes effect
 // after a reload nobody asked for.