feat(images): build, verify and publish the three images in CI; Renovate carries the digests

Operator rule 2026-10-29: image creation, hosting and lifecycle belong to CI/CD, never to a
workstation build/push. Nothing had ever been pushed to the Gitea registry.

- .gitea/workflows/images.yaml + scripts/images.sh: PRs build+verify (no credential); main
  builds, verifies and pushes toolbox, agent-runner (FROM toolbox) and conformance-runner as
  sha-<commit> + main, reading the digest back from the registry. Only changed images build.
- Each image's own build-image.sh does the build and in-image verification, now engine-neutral
  (DinD: no podman-only calls, probe over stdin not a bind mount). New toolbox build-image.sh.
- Digest references carry `# renovate-digest:` markers; renovate.json5 gains a custom.regex
  manager for them. The worker Job template is deliberately untracked (it would loop).
- renovate.yaml pins renovate-version 44.167.0: the action's default (39.264.0) rejects the
  current config's managerFilePatterns, so every run would have died at CONFIG_VALIDATION.
- Push credential: REGISTRY_PUSH_TOKEN secret; the Actions token has no package scope in
  Gitea 1.24.3 (container.go Authenticate). Retention: infra/gitea/registry-retention.sh.

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

infra/devcontainer/README.md

@@ -34,6 +34,22 @@ cd infra/devcontainer
 ```
 `ENGINE=docker ./dev.sh` to use docker instead of rootless podman.
 
+### 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
+`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
+built `FROM` that published digest (`infra/agent/versions.env`, kept current by Renovate).
+Nobody pushes this image from a workstation. To use the published one locally instead of
+building, after `podman login gitea.home.keithjackson.dev` (a token with `read:package`):
+
+```bash
+IMAGE=gitea.home.keithjackson.dev/home-network/toolbox:main ./dev.sh run make -C ci gate
+```
+
 ## Choosing a cluster — `CLUSTER=dev|prod`
 
 ```bash