Improve schedule and mackup reliability

- bin/ssr-schedule: use `launchctl bootstrap`/`bootout` (modern API) with
  legacy `load -w` fallback. The previous `load -w`-only flow silently
  no-op'd on macOS 10.15+ in some configurations, leaving `ssr status`
  reporting `inactive` after a successful `ssr schedule on`.
- bin/ssr-status: query the service via `launchctl print`/`list <label>`
  for consistent active/inactive detection.
- lib/mackup.sh: pre-materialize iCloud placeholders via `brctl download`
  and retry `mackup -f backup` up to three times to dodge the recurring
  `OSError [Errno 66] Directory not empty` race between mackup's rmtree
  and iCloud's filesystem provider. Soft-fails so the rest of `ssr sync`
  still runs even if mackup never settles; logs actionable hints.

Co-authored-by: Cursor <cursoragent@cursor.com>

bin/ssr-status

@@ -31,7 +31,9 @@ else
 fi
 
 label="${SSR_LAUNCHD_LABEL:-de.surke.ssr.sync}"
-if launchctl list 2>/dev/null | grep -q "$label"; then
+service_target="gui/$(id -u)/${label}"
+if launchctl print "$service_target" >/dev/null 2>&1 \
+   || launchctl list "$label"        >/dev/null 2>&1; then
     echo "  schedule:   active ($label)"
 else
     echo "  schedule:   inactive"