Stop failing sync over already-sent campaigns still in the repo

A campaign.md is meant to stay around after it's sent as a record of what
went out and when. Treating any non-draft campaign as a hard rejection
forced deleting it just to keep CI green, and there was no way to fail on
resolving a stale list/attachment reference either -- an archived
campaign has nothing left to resolve, so check status before doing any of
that work. Non-draft campaigns now report as a distinct, non-error
"skipped" bucket instead of "rejected".

main.go

@@ -81,6 +81,9 @@ func runSync(lm *listmonk.Client, path string) {
 	}
 	log.Printf("synced: %v", result.Synced)
 	log.Printf("unchanged: %v", result.Unchanged)
+	if len(result.Skipped) > 0 {
+		log.Printf("skipped (already sent): %v", result.Skipped)
+	}
 	if len(result.Rejected) > 0 {
 		log.Printf("rejected: %v", result.Rejected)
 	}