feat(devcontainer): bake velero CLI into the toolbox, pinned to prod server v1.18.2

The velero CLI and server are version-coupled, so VELERO_VERSION follows the
server running on prod (docker.io/velero/velero:v1.18.2, chart 12.2.3,
verified 2026-11-07). It has no renovate marker on purpose: bump it together
with the chart's appVersion. The verify stage asserts that
`velero version --client-only` reports the pin. The build arg reaches CI
through build-image.sh --print-build-args, so no .gitea/ or ci/ change is
needed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

runbooks/backup-failure.md

@@ -99,6 +99,16 @@ infra/devcontainer/dev.sh run bash -c \
      -o custom-columns=NAME:.metadata.name,BACKUP:.metadata.labels.velero\\.io/backup-name,PHASE:.status.phase,BYTES:.status.progress.bytesDone'
 ```
 
+The `velero` CLI is in the toolbox (pinned to the server version) and gives the per-item errors and
+warnings the CR only counts. Read-only; `velero backup create/delete` and `velero restore` are
+operator actions:
+
+```
+infra/devcontainer/dev.sh run bash -c \
+  'velero --kubecontext k3s-prod -n velero backup describe <name> --details; \
+   velero --kubecontext k3s-prod -n velero backup logs <name> | grep -iE "level=(error|warn)" | tail -25'
+```
+
 The plugin pin (cause 1 depends entirely on this):
 
 ```