Document the ks2 decommission plan and the new ks4 backup legs

- ks2/: decommission plan with release gates (rental ends Sep 30),
  nuc-seed and final-decommission runbooks
- ks4/local-backup-cron.md: leg-1 cron re-enabled, ks2 rsync stopgap,
  full-resend caveat when refreshes lapse past snapshots.expiry
- ks4/plakar-s3-data.md: plakar direct-to-S3 leg (auto-discovered DB
  dumps incl. grants/globals, fs sources, exclude list, restore test)
- ks4/plakar-incus-integration.md: design + scaffold status of the
  plakar incus importer
- README.md: index ks2/, update the ks4 durability bullet

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

ks2/nuc-seed.md

@@ -0,0 +1,49 @@
+# nuc pull leg — seed after FTTH
+
+Status: **prepared, waiting on the FTTH link.** Everything is already
+configured on nuc (see the main [README](../README.md)): incus remote
+`ks4` over the WireGuard tunnel (`wg-ks4`, 10.8.0.20 → 10.8.0.1),
+target pool `ks4backup` on the USB ZFS pool (`usb4t/backup/ks4`) —
+only the seed itself waited on bandwidth.
+
+## Seed (root on nuc)
+
+```sh
+# sanity: remote reachable through the tunnel
+incus list ks4: | head
+
+# full pull of every ks4 instance into pool ks4backup (screen/tmux —
+# first pass moves ~1.7 T through the WG tunnel)
+/root/scripts/incus-copy.sh -r ks4 -s ks4backup 2>&1 | tee -a /var/log/incus-copy-ks4.log
+```
+
+Notes:
+
+- First pass is a full send per instance; later refreshes are
+  ZFS-incremental **as long as they run at least every
+  `snapshots.expiry` (7 d on ks4)** — same caveat as
+  [ks4's local leg](../ks4/local-backup-cron.md).
+- Replicas arrive stopped with `boot.autostart=false` (script does
+  this) — they must never come up on the LAN with ks4's proxy devices.
+
+## Cron (after the seed)
+
+Add to nuc's root crontab, offset from the 03:30 local nucbackup copy
+and ks4's own 01:00/04:30 jobs:
+
+```cron
+0 5 * * * /root/scripts/incus-copy.sh -r ks4 -s ks4backup >> /var/log/incus-copy-ks4.log 2>&1
+```
+
+## Verification (release gate for ks2)
+
+```sh
+incus list --project backup 2>/dev/null || incus list | grep -c .   # all ks4 instances present
+# test-restore one instance: copy a replica to the default pool,
+# start it isolated, check the service answers, then delete it
+incus copy solar solar-restoretest -s default
+incus start solar-restoretest && incus exec solar-restoretest -- systemctl is-system-running
+incus delete -f solar-restoretest
+```
+
+Once verified, tick the nuc gate in the [ks2 plan](plan.md).