fix: alert on stalled rollout-checker CronJobs

[hivecommons-hive]

Finding

cluster-objects/prometheusrule.yaml alerts on the docs site's own
/api/metrics output (error rate, latency, scrape-target-down), but the
automated deploy pipeline itself had no alerting: nextra-rollout-checker
(cluster-objects/job.yaml, every 2m) and nextra-pr-rollout-checker
(cluster-objects/pr-job.yaml, every 1m) poll OCIR and roll out new
images with set -e, concurrencyPolicy: Forbid, backoffLimit: 0, and
only failedJobsHistoryLimit: 1 — a stuck/failing checker (expired
oci-config-secret, an RBAC regression on nextra-rollout-sa, an OCI API
outage) silently stops deploying new images with no signal.

Closes #7254.

Change

• Adds DocsRolloutCheckerStalled / DocsPrRolloutCheckerStalled alerts
  to cluster-objects/prometheusrule.yaml, driven by kube-state-metrics'
  kube_cronjob_status_last_successful_time (a near-universal companion
  to a Prometheus Operator install — no new exporter is added, consistent
  with every other resource in cluster-objects/).
• Adds runbooks/rollout-checker-failure.md (diagnose via kubectl get cronjob/logs, check oci-config-secret and nextra-rollout-sa RBAC)
  and a runbooks/README.md index entry.
• Extends src/__tests__/cluster-objects-metrics-consistency.test.ts to
  recognize kube-state-metrics' metric/labels as a legitimate external
  source, alongside the existing Prometheus built-in (up).

No existing alert, SLO, or probe is weakened — this only adds two new
alerts and a runbook. npx vitest run src/__tests__/cluster-objects-metrics-consistency.test.ts passes (4/4).

— hive: agent=operations backend=copilot model=claude-sonnet-4-6 copilot=1.0.88

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[kubestellar-prow]

Hi @hivecommons-hive[bot]. Thanks for your PR.

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