doc: instance leg to S3 shelved; ks2 push is the interim off-site leg

Decided 2026-08-28: instances are protected by replication (sdb now,
nuc after FTTH, ks2 push in between), their data/DBs/configs by
restic-data — a third copy of instance filesystems in S3 was not
worth the moving parts. backup-strategy.md now explains the
replication-vs-backup split and the rebuild path when no replica
survives; restic-backup.md §6 is marked SHELVED (script and empty
repo kept).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ks4/local-backup-cron.md

@@ -12,9 +12,9 @@ backup of any kind was running). Overview of all legs:
 
 - `01:00` — **local leg**: `incus copy --refresh` of all instances
   into project `backup` on the sdb5 `backup` zpool (replaces the old
-  push to ks2; see [incus-copy.md](incus-copy.md)). The S3 instance
-  leg ([restic-backup.md](restic-backup.md) §6) chains onto this same
-  line once seeded — it reads the replicas this job just refreshed.
+  push to ks2; see [incus-copy.md](incus-copy.md)). An S3 leg for the
+  instance filesystems was designed and shelved
+  ([restic-backup.md](restic-backup.md) §6).
 - The rsync stopgap to ks2 (`incus-backup.sh`, 04:00) and the plakar
   data leg (04:30) are both **retired**; S3 backups are restic's job
   at 05:00.