`common.GetDeployingAppImageName` runs `VerifyImage` and errors out when
no image has been built for the app. A freshly-created app has no image
yet, so the preview could never render for never-deployed apps. Add an
`AllowMissingImage` flag to `BuildOptions` that lets BuildAppChart
substitute a placeholder image string in that case; CommandPreview sets
it. The deploy path still passes `BuildOptions{}`, so missing-image
remains a hard error there.
Without this, the preview build invented a fresh timestamp for
`Values.global.deployment_id` and the resulting diff showed every
Deployment changing its `app.kubernetes.io/version` annotation - even when
nothing semantically changed. CommandPreview now reads the deployment id
out of `release.Chart.Metadata.Version` and passes it through a new
`BuildOptions.OverrideDeploymentID` parameter so the previewed manifests
carry the same version label as the live release. The deploy path still
generates a fresh timestamp.
Adds a `scheduler-k3s:preview <app>` command that renders a unified diff
between the manifests currently stored in the live Helm release for an app
and the manifests that the next deploy would roll out. The diff is produced
entirely via the Helm Go SDK that scheduler-k3s already uses, with no
external `helm` binary or `helm-diff` plugin install required. The
underlying diff and parse logic is vendored from `databus23/helm-diff`
(Apache 2.0) into a new internal package, trimmed of three-way-merge,
release-ownership tracking, and the non-default output formatters.
The chart-construction half of `TriggerSchedulerDeploy` has been extracted
into a reusable `BuildAppChart` helper so deploy and preview share the
same chart materialization without duplicating template logic. Secret
values are redacted by default in preview output; `--show-secrets` and
`--show-secrets-decoded` flags match the upstream `helm diff` UX. Only the
main app helm release is compared in this iteration; auxiliary releases
for config vars, image-pull secrets, and TLS certificates are out of scope
and documented as such.
