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/pm-agent/README.md

@@ -47,9 +47,9 @@ Both are deliberate, and both fail **visibly** rather than silently:
 
 1. **The image is published by CI, not by a person.** `platform/pm-agent/deployment.yaml` and
    `PM_WORKER_IMAGE` in `configmap.yaml` carry an all-zero digest placeholder until the first
-   publish. `.gitea/workflows/images.yaml` builds `agent-runner` with
-   `infra/agent/build-image.sh` (the same script, and the same in-image verification, you can
-   run locally), and on a push to `main` publishes it to
+   publish. `.gitea/workflows/images.yaml` builds `agent-runner` on rootless BuildKit
+   (`platform/buildkit/`) — the same Containerfile, and the same `verify` stage, that
+   `infra/agent/build-image.sh` builds locally — and on a push to `main` publishes it to
    `gitea.home.keithjackson.dev/home-network/agent-runner` under `sha-<commit>` and `main`.
    Renovate then follows the `main` tag through the `# renovate-digest:` marker above **both**
    lines and moves them together in one pull request. Nobody builds-and-pushes this image from