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]>

platform/pm-agent/configmap.yaml

@@ -52,12 +52,13 @@ data:
   # contract. A tag here is refused at startup (and would be DENIED by admission anyway).
   #
   # PLACEHOLDER UNTIL THE IMAGE IS PUBLISHED. The all-zero digest below is deliberately
-  # impossible: `infra/agent/build-image.sh` builds and verifies the image, the OPERATOR
-  # pushes it, and the authoritative digest from `--digestfile` replaces this value in a
-  # reviewed pull request (the same two-step the conformance-runner image follows, and for
-  # the same reason: a registry credential is not an agent's to hold). Until then the
-  # dispatcher starts, fails to pull, and says so — which is the visible failure, not a
-  # silent one.
+  # impossible. CI builds, verifies and — on main — publishes agent-runner
+  # (.gitea/workflows/images.yaml), and Renovate replaces this value with the digest the
+  # REGISTRY returned for the image's `main` tag, in a pull request that also moves the
+  # Deployment's image (same marker, same branch). Nobody pushes this image from a workstation.
+  # Until the first publish the dispatcher starts, fails to pull, and says so — which is the
+  # visible failure, not a silent one.
+  # renovate-digest: image=gitea.home.keithjackson.dev/home-network/agent-runner tag=main
   PM_WORKER_IMAGE: "gitea.home.keithjackson.dev/home-network/agent-runner@sha256:0000000000000000000000000000000000000000000000000000000000000000"
 
   # --- the model -------------------------------------------------------------