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]>

infra/devcontainer/README.md

@@ -36,9 +36,11 @@ cd infra/devcontainer
 
 ### The published toolbox
 
-`./dev.sh build` delegates to [`build-image.sh`](build-image.sh), which is the **one** build
-recipe: CI runs the same script (with its in-image verification — every tool executes, and the
-version-sensitive ones report their `versions.env` pins) and, on a push to `main` that changes
+`./dev.sh build` delegates to [`build-image.sh`](build-image.sh). The recipe is the
+`Containerfile` plus that script's build-arg list, and the proof is the Containerfile's `verify`
+stage (every tool on `PATH`, the version-sensitive ones report their `versions.env` pins, no
+network). CI builds the same Containerfile on rootless BuildKit with the args from
+`build-image.sh --print-build-args`, and, on a push to `main` that changes
 `Containerfile`, `versions.env` or `build-image.sh`, **publishes** the result as
 `gitea.home.keithjackson.dev/home-network/toolbox` (tags `sha-<commit>` and `main`) —
 see [`.gitea/workflows/images.yaml`](../../.gitea/workflows/images.yaml). `agent-runner` is