system_df:
Assembles disk-usage report from SYNO.Docker.Image/list and
SYNO.Docker.Container/list. Reports image count/size/reclaimable
(images not referenced by any container), container running/stopped.
Gracefully degrades when one API is unavailable.
system_prune:
Without confirmed=True: lists dangling/unused images and stopped
containers with sizes (dry-run preview).
With confirmed=True: calls SYNO.Docker.Utils/prune and reports
reclaimed space from the response (SpaceReclaimed field).
10 unit tests: stats counts, reclaimable detection, preview content,
confirmed execution, missing-response-field graceful handling, API error.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
