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]>

conformance/incluster/README.md

@@ -61,10 +61,13 @@ credentials — the Job's own scoped ServiceAccount does (R-SEC-02).
   > digest-pinned to the conformance-runner image, `command` exactly
   > `/usr/local/bin/conformance-clone`, and **no `args` at all**. `ephemeralContainers` and
   > `env`/`envFrom` were the same shape of hole and are closed in the same policy.
-- `build-image.sh` — builds the above, **verifies the toolchain by running it** inside the
-  image as uid 65532 with `--read-only`, and prints the digest. It **never pushes**: it
-  prints the exact `podman push --digestfile …` command for the operator instead. See
-  `conformance/versions.env` `CONFORMANCE_RUNNER_IMAGE` for the post-push pin.
+- `build-image.sh` — builds the above and **verifies the toolchain by running it** inside the
+  image as uid 65532 with `--read-only`. It **never pushes**, and neither does anyone from a
+  workstation: CI (`.gitea/workflows/images.yaml`) runs this same script, and on a push to
+  `main` publishes the verified image under `sha-<commit>` and `main`. Renovate then moves
+  `CONFORMANCE_RUNNER_IMAGE_DIGEST` in `conformance/versions.env` onto the registry-returned
+  digest in a pull request (that file is a sensitive path, so the bump needs an
+  administrator's `/approve-override`).
   `--verify-clone` additionally has `conformance-clone` really clone this repo at a real SHA
   and asserts the checkout matches (opt-in, because it needs the Gitea host reachable — the
   default verify stays offline so an air-gapped build is fully checked, not half-checked).