k8s-version-upgrade: scope chain-fail alert to terminal reasons + sync docs

Refines the new K8sUpgradeChainJobFailed alert from a bare failed-pod count to
the terminal job-condition reasons (BackoffLimitExceeded|DeadlineExceeded). A
phase whose first pod failed but whose retry SUCCEEDED must NOT fire: every
firing alert also halts kured, so a bare-count false-positive would block all
OS node reboots for the Job's 7-day TTL. Verified against kube-state-metrics:
the stuck preflight reports reason="BackoffLimitExceeded"; a Complete job has 0
for the terminal reasons.

Docs updated to match the behaviour change (per the same-commit docs rule):
  - docs/runbooks/k8s-version-upgrade.md — new alert in the gates list; the
    "kill a stuck Job" recovery now leads with retry-on-failure self-heal.
  - docs/architecture/automated-upgrades.md — fourth Upgrade Gates alert;
    retry-on-failure note on the deterministic-naming paragraph.
  - .claude/skills/upgrade-state/SKILL.md — new "chain failed" status, legend
    entry, and drill-down (also copied to the active ~/.claude copy).

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

.claude/skills/upgrade-state/SKILL.md

@@ -61,8 +61,11 @@ Pushgateway (`prometheus-prometheus-pushgateway.monitoring:9091`):
 - `k8s_upgrade_in_flight` — 0/1
 - `k8s_upgrade_started_timestamp` — when the current chain started (0 when idle)
 
-`K8sUpgradeStalled` alert fires when `in_flight=1` and the chain has
-been running >90 minutes. The script raises `✗` in the same window.
+`K8sUpgradeStalled` fires when `in_flight=1` and the chain has been running
+>90 minutes. `K8sUpgradeChainJobFailed` fires when a phase Job terminally
+failed — including a **preflight that aborted before `in_flight` was set**
+(the gates exit pre-metric). The script raises `✗` for either, and reads the
+Jobs directly, so it also catches a Failed preflight that left no metric.
 
 ## Status-icon legend
 
@@ -72,7 +75,7 @@ been running >90 minutes. The script raises `✗` in the same window.
 | `→` | Update available, not yet applied (K8s patch/minor) |
 | `…` | In flight — chain currently running |
 | `⚠` | Attention: held-with-bumps, recent errors, pending approvals |
-| `✗` | Broken: pod down, alert firing, chain stalled |
+| `✗` | Broken: pod down, alert firing, chain stalled, or a chain Job failed |
 
 ## Drill-down — when a row trips, what to do
 
@@ -177,6 +180,31 @@ kubectl -n monitoring exec deploy/prometheus-server -c prometheus-server -- sh -
        --header='Content-Type: text/plain'"
 ```
 
+### K8s `✗ chain failed` — a phase Job terminally failed
+
+`K8sUpgradeChainJobFailed` would fire. Most often a **preflight** that aborted
+on a gate (a critical alert firing, a node not Ready, a kubeadm-plan mismatch) —
+these exit before `in_flight` is set, so `K8sUpgradeStalled` never sees them, and
+the deterministic name + 7d TTL blocked re-spawn (the 2026-06-12 5-day wedge).
+
+```bash
+kubectl -n k8s-upgrade get jobs
+kubectl -n k8s-upgrade describe job <failed-job>    # check the Failed reason
+# Preflight abort reasons post to Slack ONLY (not stdout), so Loki won't have
+# them. Replay the gate instead — which critical alerts were firing at the
+# failure time? (ALERTS{severity="critical"} in Prometheus, query at that ts.)
+```
+
+Recovery is now mostly automatic: the detection CronJob and `spawn_next`
+re-spawn a terminally-Failed Job on the next cycle (retry-on-failure), so a
+transient gate clears within ~24h. To expedite, delete the Failed Job and
+trigger detection:
+
+```bash
+kubectl -n k8s-upgrade delete job <failed-job>
+kubectl -n k8s-upgrade create job --from=cronjob/k8s-version-check manual-detect-$(date +%s)
+```
+
 ### K8s `✗ detection stale` — last detection >9 days
 
 ```bash

docs/architecture/automated-upgrades.md

@@ -274,8 +274,13 @@ Job 6 — postflight      (no pinning)
 Each Job runs `scripts/upgrade-step.sh`, which dispatches on `$PHASE` and ends
 by spawning the next Job (`envsubst < /template/job-template.yaml | kubectl
 apply -f -`). Job names are deterministic (`k8s-upgrade-<phase>-<target_version>[-<node>]`)
-so `apply` reconciles to a single Job per run — re-running a failed Job
-won't duplicate downstream Jobs.
+so `apply` reconciles to a single Job per run — re-running won't duplicate
+downstream Jobs. The detection CronJob and `spawn_next` additionally delete +
+re-spawn a terminally-**Failed** Job of the same name (rather than skipping it
+on existence), so a transient preflight gate self-heals on the next cycle
+instead of wedging the pipeline until the dead Job's 7d TTL expires
+(retry-on-failure, added 2026-06-17 after a spurious critical alert stalled
+1.34.9 for 5 days).
 
 ### Self-preemption history (the reason for the Job-chain rewrite)
 
@@ -305,10 +310,11 @@ each Job's pod and its drain target are always different nodes.
 - **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.
-- **Three Upgrade Gates alerts**:
+- **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.
   - `K8sUpgradeStalled` — `k8s_upgrade_in_flight==1 && time()-k8s_upgrade_started_timestamp > 5400` for 5m. Catches a chain Job dying without spawning its successor.
+  - `K8sUpgradeChainJobFailed` — `kube_job_status_failed{namespace="k8s-upgrade",job_name=~"k8s-upgrade-.*",reason=~"BackoffLimitExceeded|DeadlineExceeded"} > 0` for 15m (warning). Catches a phase Job that terminally failed **before `in_flight` was set** (the preflight gates exit pre-metric) — invisible to the two `in_flight`-based alerts above; this was the blind spot behind the 5-day 1.34.9 preflight wedge. Reason-scoped so a retry-success doesn't false-positive (and so it doesn't needlessly block kured).
 - **Pushgateway metrics**:
   - `k8s_upgrade_in_flight` (set in preflight, cleared in postflight)
   - `k8s_upgrade_snapshot_taken` (set after etcd snapshot Job completes with ≥1 KiB)

docs/runbooks/k8s-version-upgrade.md

@@ -115,7 +115,8 @@ Pushed by upgrade-step.sh during phase execution; observed by the
 - **`K8sVersionSkew`** — distinct 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 Stage 2 failing silently.
 - **`K8sUpgradeStalled`** — `k8s_upgrade_in_flight==1 && time()-k8s_upgrade_started_timestamp > 5400` for 5m. Catches a Job in the chain dying without spawning its successor.
-- All three alerts ALSO block kured (same `--prometheus-url` halt-on-alert mechanism) so the OS-reboot pipeline can't run on top of a half-done version upgrade.
+- **`K8sUpgradeChainJobFailed`** — `kube_job_status_failed{namespace="k8s-upgrade",job_name=~"k8s-upgrade-.*",reason=~"BackoffLimitExceeded|DeadlineExceeded"} > 0` for 15m (warning). Catches a phase Job that **terminally failed before `k8s_upgrade_in_flight` was set** — the preflight gates exit pre-metric, so the two `in_flight`-based alerts above are blind to a failed preflight (this is what hid the 5-day 1.34.9 wedge on 2026-06-12). Reason-scoped to terminal job conditions so a retry-success doesn't false-positive (a bare failed-pod-count would otherwise also block kured for the Job's 7d TTL).
+- All four alerts ALSO block kured (same `--prometheus-url` halt-on-alert mechanism) so the OS-reboot pipeline can't run on top of a half-done version upgrade.
 
 ### Vault secrets
 
@@ -202,8 +203,18 @@ EOF
 ```
 
 ### Kill a stuck Job (chain halted mid-flight)
-The chain stalls if any Job dies without spawning its successor. `K8sUpgradeStalled`
-fires after 90 min. Recovery:
+A phase Job that dies without spawning its successor halts the chain. Two alerts
+surface it: `K8sUpgradeStalled` (a mid-chain Job that died with `in_flight=1`,
+after 90 min) and `K8sUpgradeChainJobFailed` (any phase that terminally failed,
+after 15 min — including a **preflight** that aborted before `in_flight` was set,
+which `K8sUpgradeStalled` cannot see).
+
+**Preflight failures now self-heal** (since 2026-06-17): the detection CronJob and
+`spawn_next` delete + re-spawn a terminally-Failed Job instead of skipping it on
+name-existence (retry-on-failure), so a transient preflight gate — e.g. a spurious
+critical alert like the ttyd web-terminal probe that wedged 1.34.9 for 5 days —
+clears on the next daily cycle. A mid-chain phase that keeps failing still needs
+manual recovery: fix the root cause, then:
 
 ```bash
 # 1. Identify the failed Job

stacks/monitoring/modules/monitoring/prometheus_chart_values.tpl

@@ -2224,29 +2224,29 @@ serverFiles:
               severity: critical
             annotations:
               summary: "K8s upgrade has been in flight for >90 min — chain is stuck. Check: kubectl -n k8s-upgrade get jobs"
-          # K8sUpgradeChainJobFailed: catches a FAILED phase Job even when it
-          # aborts BEFORE pushing k8s_upgrade_in_flight=1. The preflight gates
-          # (nodes-ready, halt-on-alert, settle-window, kubeadm-plan) all exit
-          # pre-metric, so a failed preflight is invisible to K8sUpgradeStalled
-          # and EtcdPreUpgradeSnapshotMissing (both need in_flight=1) AND to
-          # upgrade_state.sh — exactly how a transient critical alert wedged the
-          # 1.34.9 preflight for 5 days (2026-06-17). With the retry-on-failure
-          # idempotency guard the next detection cycle deletes + re-spawns it, so
-          # this firing for 15m means it re-failed: investigate the root cause.
-          # NB: keyed on failed-pod count (bare >0, matching the file's other
-          # job-failure alerts) not the terminal Failed *condition* — so a phase
-          # whose 1st pod failed but whose retry succeeded keeps this firing until
-          # the Job's 7d TTL expires. Accepted: warning-only + alert-on-change
-          # (notifies once) + send_resolved, and upgrade_state.sh uses the precise
-          # Failed condition. A false-positive here beats missing a real wedge.
+          # K8sUpgradeChainJobFailed: catches a TERMINALLY-failed phase Job even
+          # when it aborts BEFORE pushing k8s_upgrade_in_flight=1 (the preflight
+          # gates — nodes-ready, halt-on-alert, settle-window, kubeadm-plan — all
+          # exit pre-metric). K8sUpgradeStalled and EtcdPreUpgradeSnapshotMissing
+          # both need in_flight=1, and upgrade_state.sh was metric-blind too, so a
+          # failed preflight was invisible: exactly how a transient critical alert
+          # wedged 1.34.9 for 5 days (2026-06-12). Scoped to the terminal
+          # job-condition reasons (BackoffLimitExceeded/DeadlineExceeded), NOT a
+          # bare failed-pod count, so a phase whose 1st pod failed but whose retry
+          # SUCCEEDED does not fire — important because every firing alert also
+          # halts kured (OS-reboot pipeline), and a bare-count false-positive would
+          # block all node reboots for the Job's 7d TTL. With the retry-on-failure
+          # idempotency guard the next detection cycle deletes + re-spawns the
+          # Failed Job (clearing this within ~24h); a sustained firing means it
+          # re-failed — investigate the root cause.
           - alert: K8sUpgradeChainJobFailed
-            expr: kube_job_status_failed{namespace="k8s-upgrade", job_name=~"k8s-upgrade-.*"} > 0
+            expr: kube_job_status_failed{namespace="k8s-upgrade", job_name=~"k8s-upgrade-.*", reason=~"BackoffLimitExceeded|DeadlineExceeded"} > 0
             for: 15m
             labels:
               severity: warning
               subsystem: k8s-upgrade
             annotations:
-              summary: "K8s upgrade chain Job {{ $labels.job_name }} has failed pods — pipeline likely wedged. kubectl -n k8s-upgrade get jobs ; kubectl -n k8s-upgrade describe job {{ $labels.job_name }}"
+              summary: "K8s upgrade chain Job {{ $labels.job_name }} terminally failed ({{ $labels.reason }}) — pipeline wedged. kubectl -n k8s-upgrade get jobs ; kubectl -n k8s-upgrade describe job {{ $labels.job_name }}"
       - name: "Traefik Ingress"
         rules:
           - alert: TraefikDown