Stage 5: installer chart + release infra, kind e2e pass through the chart

Chart (charts/terdut-operator) generated via kubebuilder's own helm/v2-alpha
plugin from config/'s kustomize output -- CRDs + manager Deployment/RBAC
come from the same markers every other stage already generates, one source
of truth. Hand-added on top: the optional terdutServer values block
(DESIGN.md §10's "helm install and get a server" path, off by default) and
the release-skill plumbing -- .release.conf, release-vars/helm-lint/push/
helm-package/helm-push/release Makefile targets, .gitea/workflows/release.yaml
(test -> image/chart -> scan-image) -- mirroring terdut-server's own shape
(registry/namespace convention, multi-arch buildx push, trivy/govulncheck/
gitleaks scans). ci.yaml gains security and chart jobs to match.

Two real issues caught while wiring this, fixed before either shipped:
- Dockerfile's builder stage didn't pin --platform=$BUILDPLATFORM, which
  would have made a multi-arch release build fail outright on this org's
  runners (no binfmt registration) -- same fix terdut-server's own
  Dockerfile already needed for the same reason.
- govulncheck found one real, reachable finding: google.golang.org/grpc
  v1.82.1 (transitive via controller-runtime's otel exporter), fixed by
  bumping to v1.83.1.

Full golden-path kind e2e pass, this time through `helm install` rather than
raw kustomize: TerdutServer (real terdut-server v0.33.0 image) -> TerdutTeam
-> one of each child kind, each confirmed Ready and then independently
confirmed against terdut-server's own API from inside the cluster (not just
the operator's own status). Deleted every CR in reverse order and confirmed
server-side cleanup the same independent way for all three child kinds, the
team, and the server. No new bugs found -- Stage 1's own kind pass already
caught what a real cluster catches that envtest can't.

Also dropped the kubebuilder helm plugin's default .github/workflows/
scaffold, same as Stage 0 already did for the main scaffold: this org runs
on Gitea, not GitHub.

Not done here, deliberately: an actual tagged release. release-preflight
found no terdut-operator/ entry under Ryuvia/charts yet to bump -- that
one-time wrapper bootstrap is a decision about deploying this operator for
real, not a side effect of finishing this stage.

make fmt lint test helm-lint build all clean.

go.mod

@@ -12,7 +12,7 @@ require (
 )
 
 require (
-	cel.dev/expr v0.25.1 // indirect
+	cel.dev/expr v0.25.2 // indirect
 	github.com/Masterminds/semver/v3 v3.4.0 // indirect
 	github.com/antlr4-go/antlr/v4 v4.13.1 // indirect
 	github.com/beorn7/perks v1.0.1 // indirect
@@ -88,7 +88,7 @@ require (
 	gomodules.xyz/jsonpatch/v2 v2.4.0 // indirect
 	google.golang.org/genproto/googleapis/api v0.0.0-20260526163538-3dc84a4a5aaa // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20260526163538-3dc84a4a5aaa // indirect
-	google.golang.org/grpc v1.82.1 // indirect
+	google.golang.org/grpc v1.83.1 // indirect
 	google.golang.org/protobuf v1.36.12-0.20260120151049-f2248ac996af // indirect
 	gopkg.in/evanphx/json-patch.v4 v4.13.0 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect