doc: split per-host READMEs, gitea cross-repo links, consistency pass

- nuc/README.md and ks4/README.md carry the host sections (+ network
  flows) that lived in the top-level README; links rebased
- top README: repo links (doc/scripts on git.lutran.fr), index points
  at the new per-host pages
- cross-repo references now use https://git.lutran.fr/julien/scripts
  instead of relative ../scripts paths that resolve nowhere
- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED
  / RETIRED with pointers to restic-backup.md; their measurements and
  rationale kept
- install.md, local-backup-cron.md, incus-copy.md: crontab sections
  updated to the live schedule (01:00 replicas, 05:00 restic, Sun
  maintenance); retired legs labelled as such
- restic-backup.md: status live, cutover recorded, post-GC memory
  estimate, seed plan dated
- seafile-gc.md: online GC noted, stale 'crons commented out' removed
- ks2/: what-ks2-does-today rewritten (nothing writes to it any more),
  legs table and gates reflect restic, decommission steps updated
- db-exclude replaces the plakar-era config name (script keeps a
  fallback)

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

ks4/plakar-incus-integration.md

@@ -1,4 +1,14 @@
-# plakar incus integration — design notes
+# plakar incus integration — design notes (RETIRED)
+
+> **Historical.** The importer works (smoke-tested against ks4's
+> `backup` project) but production moved to restic, which needs no
+> plugin: `incus file mount` exposes any instance — including stopped
+> replicas — over the same per-instance sftp API
+> ([restic-backup.md](restic-backup.md) §6). Code lives on at
+> <https://github.com/jlutran/plakar-integration-incus>; the analysis
+> below (per-file vs image, replica-vs-live consistency, the
+> seek-bound wall) is what shaped the restic design.
+
 
 Status: **importer scaffolded and compiling 2026-08-23** in
 `perso/plakar/integration-incus` (walker modeled on the official sftp