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]>

infra/devcontainer/versions.env

@@ -11,8 +11,8 @@ OPENTOFU_VERSION=1.9.0
 AGE_VERSION=1.2.1
 SOPS_VERSION=3.9.4
 FLUX_VERSION=2.4.0
-# Aligned with ci/versions.env (KUBECTL=1.31.4) and the k3s 1.31.5 cluster.
-KUBECTL_VERSION=1.31.4
+# Aligned with ci/versions.env (KUBECTL=1.36.4) and the k3s 1.36.4 cluster.
+KUBECTL_VERSION=1.36.4
 HELM_VERSION=3.16.0
 
 # --- Conformance test toolchain (match ci/versions.env) ----------------------