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]>

platform/velero/README.md

@@ -9,7 +9,7 @@
 
 ## What this is (LIVE)
 
-Flux `HelmRelease` (Velero chart 8.x / Velero 1.15.x) wired to the on-Proxmox **MinIO** S3 target:
+Flux `HelmRelease` (Velero chart 12.x / Velero 1.18.x) wired to the on-Proxmox **MinIO** S3 target:
 
 - **Endpoint:** `http://192.168.1.9:9000` (plain HTTP, **path-style**, region `us-east-1` placeholder)
 - **BucketStorageLocation `default`:** provider `aws`, bucket `velero`, `s3ForcePathStyle=true`.