Fix snapshot rotation failure on remote filesystems

Snapshots with read-only permissions (from chmod -w at creation) and
internal dirs with restrictive modes (e.g. /etc at 555) prevented
rm -rf from deleting old snapshots during rotation. This caused
cascading mv failures and silent rotation breakage.

- Add chmod -R u+w before rm -rf of oldest snapshot
- Clean up orphan snapshots beyond retention count
- Add error checking on rm, mv, and cp -al operations

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