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]>

clusters/base/kustomization.yaml

@@ -98,4 +98,5 @@ resources:
   # deliberate operator kernel upgrade creates — the human gate is the sentinel, not the daemon.
   # See clusters/base/kured.yaml and platform/kured/helmrelease.yaml.
   - kured.yaml           # Flux Kustomization -> ./platform/kured (dependsOn platform)
+  - buildkit.yaml        # Flux Kustomization -> ./platform/buildkit (dependsOn platform)
   - cluster-vars.yaml    # overlay knob surface (base defaults)