k8s-version-upgrade: unblock 1.34.9 — skip kubeadm CoreDNS addon + busybox-date fix

The 1.34.9 master upgrade hard-failed `kubeadm upgrade apply` preflight: CoreDNS
is at v1.12.4 (Keel auto-bumped it 1.12.1 -> 1.12.4 on 2026-05-26 via a stale
kube-system out-of-band annotation), and 1.12.4 is ahead of kubeadm 1.34.9's
bundled corefile-migration table ("start version not supported").

- scripts/update_k8s.sh: master `kubeadm upgrade apply` now runs with
  `--ignore-preflight-errors=CoreDNSMigration,CoreDNSUnsupportedPlugins
  --skip-phases=addon/coredns`. A dry-run proved --ignore ALONE would overwrite
  our custom split-horizon Corefile with kubeadm's default AND downgrade the
  image; --skip-phases leaves CoreDNS 100% untouched while the control plane
  upgrades. CoreDNS is pinned off Keel (keel.sh/policy=never) to stop the drift.
- stacks/k8s-version-upgrade/scripts/upgrade-step.sh: fix the preflight
  quiet-baseline (settle-window) check, which silently no-op'd on the ghcr
  claude-agent-service image's busybox `date` (can't parse ISO8601). Now tries
  GNU then busybox `-D`, and warns+skips on parse failure (no silent fail-open).
- docs: runbook + architecture document the CoreDNS handling.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/architecture/automated-upgrades.md

@@ -309,7 +309,11 @@ each Job's pod and its drain target are always different nodes.
   ConfigMap, and a `template` ConfigMap into each Job pod.
 - **Per-node script**: `infra/scripts/update_k8s.sh`. Caller passes
   `--role master|worker --release X.Y.Z`. Piped via SSH into each node by
-  upgrade-step.sh.
+  upgrade-step.sh. The master path runs `kubeadm upgrade apply` with
+  `--ignore-preflight-errors=CoreDNSMigration,CoreDNSUnsupportedPlugins
+  --skip-phases=addon/coredns` so kubeadm never touches CoreDNS (custom Corefile
+  + separately-tracked image; CoreDNS is pinned off Keel via `keel.sh/policy=never`).
+  See the runbook's "CoreDNS is NOT upgraded by kubeadm here".
 - **Four Upgrade Gates alerts**:
   - `K8sVersionSkew` — kubelet/apiserver `gitVersion` count >1 for 30m. Catches a half-done rollout.
   - `EtcdPreUpgradeSnapshotMissing` — `k8s_upgrade_in_flight==1 && k8s_upgrade_snapshot_taken==0` for 10m. Catches preflight failing silently.