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]>

.gitea/workflows/README.md

@@ -115,14 +115,47 @@ CI. That is a real coverage gap, not a phase.
 | variable | `REQUIRE_CLUSTER_TESTS` | `'true'` (Phase 1+) makes unverified cluster-needing touched reqs a hard block |
 | secret | `CONFORMANCE_TRIGGER_KUBECONFIG` | narrow creds to create/await the in-cluster Chainsaw Job (not cluster-admin) |
 
+## How images are built
+
+This repo's three images — `toolbox` (`infra/devcontainer/`), `agent-runner` (`infra/agent/`,
+built `FROM` the toolbox) and `conformance-runner` (`conformance/incluster/`) — are built,
+verified and published by [`images.yaml`](images.yaml), never from a workstation
+(operator rule, 2026-10-29). The logic is in [`scripts/images.sh`](scripts/images.sh); each
+image is built by **its own** `build-image.sh`, the same script a developer runs locally, so the
+in-image verification (non-root uid, read-only rootfs, tool presence, `PM_SELFTEST=1
+pm-dispatch`, the `conformance-clone` refusals) exists once.
+
+| Trigger | What happens |
+|---|---|
+| pull request touching an image's inputs | build + verify only. Nothing is pushed and **no registry credential is referenced** |
+| push to `main` touching an image's inputs | build + verify + publish to `gitea.home.keithjackson.dev/home-network/<image>` as `sha-<commit>` (immutable) and `main` (moving); the **registry-returned** digest is in the job summary |
+| `workflow_dispatch` | the same as a push when run on `main` (all images by default); verify-only elsewhere |
+
+Only images whose inputs changed are built. The publish job **never writes to git**: Renovate
+follows each image's `main` tag through a `# renovate-digest:` marker next to every digest
+reference (`platform/pm-agent/`, `infra/agent/versions.env`, `conformance/versions.env`) and
+opens the bump PR, which goes through the gate like anything else. A PR opened with the Actions
+token would not trigger the required gate, which is why it is Renovate's job and not this one's.
+
+Credential: the repository secret **`REGISTRY_PUSH_TOKEN`** (scope `write:package`, owned by an
+account with package write on the org). The automatic Actions token cannot push in Gitea 1.24.3
+— the reasoning, with source citations, is in the header of `images.yaml`. Retention:
+[`infra/gitea/registry-retention.sh`](../../infra/gitea/registry-retention.sh), an operator
+action, keeps every digest git pins plus the newest tags.
+
+Cost on this runner: the job shares the single act_runner slot with the gate, and the DinD store
+is a node `emptyDir`; the job prunes it on exit.
+
 ## Files
 
 ```
 .gitea/workflows/
   merge-gate.yaml            the pipeline (lint → l0 → l1 → conformance → decision)
+  images.yaml                build/verify/publish this repo's container images (see above)
   scripts/
     install-tools.sh         installs the pinned tools per stage (from ci/versions.env)
     decide.sh                the R-TEST-03 merge decision over job results
+    images.sh                the image pipeline's driver (what to build, base, push, digest)
   README.md                  this file
 ```