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>

ks2/plan.md

@@ -61,9 +61,9 @@ when nuc takes over.
 2. ~~[local-backup-cron.md](../ks4/local-backup-cron.md)~~ — **done
    2026-08-22**: leg 1 cron re-enabled, ks2 rsync kept as stopgap;
    `livetrail` verified present in the `backup` project
-3. ~~plakar S3 leg~~ → **replaced by restic**
-   ([restic-backup.md](../ks4/restic-backup.md), live 2026-08-28;
-   plakar's own doc kept as history)
+3. ~~first S3 leg~~ → **restic** is the S3 tool
+   ([restic-backup.md](../ks4/restic-backup.md), live 2026-08-28; the
+   predecessor's doc is kept as reference)
 4. ~~instance leg to S3~~ — **shelved 2026-08-28**: instances are
    protected by replication (sdb + nuc/ks2), their data and configs by
    `restic-data`