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/agent/versions.env

@@ -9,10 +9,16 @@
 # truth and, the first time the two drifted, the worker's "green" would stop meaning the
 # gate's green. So this file pins ONLY what the toolbox does not carry.
 
-# The toolbox image the runner is layered on. Built by `infra/devcontainer/dev.sh build`
-# (tag `home-k3s-dev:local`); infra/agent/build-image.sh asserts it exists rather than
-# silently building a different base.
-AGENT_BASE_IMAGE=home-k3s-dev:local
+# The toolbox image the runner is layered on: the PUBLISHED toolbox, by digest. CI builds and
+# publishes it (.gitea/workflows/images.yaml) and Renovate moves this line onto each new digest
+# of its `main` tag via the marker below, in a pull request — nobody edits it by hand.
+#
+# ALL-ZERO UNTIL THE FIRST PUBLISH. The first run of the image pipeline on main sees this
+# placeholder, builds and publishes the toolbox first, and builds agent-runner FROM that fresh
+# digest. For a LOCAL build on a local toolbox, override it:
+#   infra/devcontainer/dev.sh build && AGENT_BASE_IMAGE=home-k3s-dev:local infra/agent/build-image.sh
+# renovate-digest: image=gitea.home.keithjackson.dev/home-network/toolbox tag=main
+AGENT_BASE_IMAGE=gitea.home.keithjackson.dev/home-network/toolbox@sha256:0000000000000000000000000000000000000000000000000000000000000000
 
 # Node.js — the runtime for the Claude Code CLI, installed from the Fedora base's module
 # stream. A major version, not a patch: the CLI supports the active LTS line and the