feat(images): build on rootless BuildKit over mTLS; verification moves into Containerfile verify stages

Operator decision on PR #81: no CI job may reach a privileged daemon. The act runner is untouched.

- platform/buildkit (+ clusters/base/buildkit.yaml): moby/buildkit v0.34.1-rootless pinned by
  digest, uid 1000, not privileged; caps drop ALL + SETUID/SETGID, seccomp/AppArmor Unconfined,
  read-only rootfs (each measured under podman). cert-manager CA + server/client certs, ClusterIP
  :1234, default-deny with ingress only from the act-runner pod (DinD masquerades jobs to it).
  No Kyverno exception: every applied policy matches app-namespace=true only.
- Each Containerfile names its shipped stage and ends in a `verify` stage holding every former
  `docker run` assertion (uid 65532, RUN --network=none, writable-path inventory standing in for
  the read-only rootfs). The old `&& echo` checks that could never fail are real assertions now.
- images.sh drives buildkitd with a pinned buildx (sha256-checked) via bake; PRs build verify
  targets with agent-runner FROM the in-run toolbox (target: context); main verifies, then pushes.
  Fails loudly when the builder secrets are absent or buildkitd is unreachable.
- infra/gitea/buildkit-client-secret.sh copies the client cert into Actions secrets.

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

platform/buildkit/kustomization.yaml

@@ -0,0 +1,13 @@
+---
+# platform/buildkit — rootless BuildKit, the CI image builder. Reconciled by the Flux
+# Kustomization `buildkit` (clusters/base/buildkit.yaml). See README.md.
+# versions.env is NOT a resource: it pins the CI-side client (buildx) and the address CI dials.
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - namespace.yaml
+  - certificates.yaml
+  - configmap.yaml
+  - deployment.yaml
+  - service.yaml
+  - networkpolicy.yaml