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]>

docs/plans/ssd-etcd-isolation.md

@@ -1,5 +1,12 @@
 # SSD-backed etcd isolation for the k3s dev cluster
 
+> ## ✅ COMPLETED / historical — executed 2026-08-30, see git log
+>
+> **This proposal was approved and carried out.** Each node now has a dedicated `virtio2` SSD etcd
+> disk (`/dev/vdc`, `local-zfs`) hosting the etcd data dir. The **Status: PROPOSAL** line below and
+> the sizing/RAM figures are the original investigation record — retained for the reasoning, not as a
+> live to-do. Current state is tracked in `infra/PHASE1-STATUS.md` and the 2026-08-30 DECISIONS-LOG entry.
+
 **Status:** PROPOSAL — awaiting user approval. No changes have been made.
 **Author:** investigation agent (read-only), 2026-08-29
 **Problem:** Longhorn replica-rebuild I/O storms on the HDD-backed VM disks starve