Checkpoint rewind gains a documented skippedLinks telemetry/response field for symlink-safety refusals

Checkpoint rewind responses now document a skippedLinks field, counting tracked files that weren't restored because a symlink or hard link was found at the tracked path, a parent directory no longer resolves correctly, or the backup couldn't be safely read. This field is only populated on a real (non-dryRun) rewind; dryRun previews never set it and their counts don't reflect link-safety refusals.