docs: reconcile docs/comments with post-upgrade cluster state

Text-only cleanup after the upgrade campaign. Updates stale version
strings, prose, and comments to the verified current facts; no
Kubernetes behavior or version pins changed except the kubectl
toolchain bump (below).

Verified facts applied: k3s v1.36.4+k3s1, Debian 13 trixie,
Cilium 1.20.1 / Gateway API v1.6.1, Longhorn 1.12.1, node RAM 24 GiB,
dedicated virtio2 SSD etcd disk per node, Velero chart 12.x / v1.18,
Grafana 13.2.0.

- Version strings across infra/ and platform/ READMEs + comments
- kubectl toolchain 1.31.4 -> 1.36.4 (devcontainer/ci/conformance
  versions.env; 1.31 vs a 1.36 cluster exceeded the +-1 skew)
- Retired-single-apps-Kustomization refs -> apps-wave-0/1/2 + apps-vms
  with per-wave SOPS decryption (flux/clusters docs + bootstrap.sh)
- COMPLETED/historical banners on the three executed plan docs
- Resolved TODO(WS-SEC) markers -> Renovate-maintained ranges
- Appended dated DECISIONS-LOG entry (2026-08-30) superseding the
  2026-08-26 LIVE STATE line

Left as revalidation items (not blind-edited): external-snapshotter
v8.2.0 vs 1.36, Cilium ext_authz/forward-auth relevance under 1.20.1,
conformance ImageVolume feature-gate assertions under 1.36.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

clusters/README.md

@@ -29,8 +29,9 @@ clusters/
 
 ## How the overlay model satisfies R-ENV-02
 
-The base authors the reconciliation graph **once**. `platform.yaml` and `apps.yaml` (Flux
-`Kustomization`s) both declare `postBuild.substituteFrom` the `cluster-vars` ConfigMap. Platform and
+The base authors the reconciliation graph **once**. `platform.yaml` and the app-wave Kustomizations
+(`apps-wave-0/1/2.yaml` — the single `apps.yaml` was retired; `apps-vms.yaml` deliberately omits it,
+see below) declare `postBuild.substituteFrom` the `cluster-vars` ConfigMap. Platform and
 app manifests use `${TOKEN}` placeholders for every value that legitimately differs between
 environments (issuer, replica counts, storage sizes, retention, node placement).