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/probe-validation.sh

@@ -22,7 +22,9 @@
 ###############################################################################
 set -uo pipefail
 
-HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# `:-$0` because build-image.sh feeds this script over STDIN (`bash -s`), where BASH_SOURCE is
+# empty and `set -u` would abort on it; PM_WORKER_PATH is always set in that case.
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]:-$0}")" && pwd)"
 WORKER="${PM_WORKER_PATH:-${HERE}/pm-worker}"
 [ -r "$WORKER" ] || { echo "cannot read ${WORKER}" >&2; exit 2; }