doc: plakar out of the current architecture, kept as reference

backup-strategy.md now leads with the three target legs (local incus
push to sdb, nuc pull over WireGuard, restic to S3 for DBs and
selected trees) and carries the plakar/ks2 story in a closing History
section. restic-backup.md, install.md, local-backup-cron.md and the
ks2 docs describe the predecessor only as 'the first S3
implementation', with one operational note that plakar is still
installed for issue #2338 reproduction.

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

ks4/install.md

@@ -164,9 +164,10 @@ non-mirrored `data` pool:
   into the `restic-data` bucket ([restic-backup.md](restic-backup.md)).
 - **Sun 14:00 `restic-maintenance.sh`** → prune + integrity checks.
 
-Historical (retired 2026-08): the `ks2` push and `incus-backup.sh`
-rsync leg, and the plakar data leg
-([plakar-s3-data.md](plakar-s3-data.md)).
+A `ks2` interim replica push (`incus-copy.sh -d ks2 -m push`, 02:00)
+runs until the nuc pull leg is seeded. Retired 2026-08: the
+`incus-backup.sh` rsync leg and the first S3 implementation
+([plakar-s3-data.md](plakar-s3-data.md) — reference only).
 
 - **01:00 — `incus-copy.sh` → `ks2`**: differential `incus copy … \
   --refresh --refresh-exclude-older --mode push` of every *running*