feat(ci): run the merge gate inside the CI-built toolbox image, pinned by digest

The gate job now runs in gitea.home.keithjackson.dev/home-network/toolbox@sha256:83a3501e...
(the image images.yaml publishes from infra/devcontainer/Containerfile) instead of
catthehacker/ubuntu:act-22.04 plus a per-run download of yamllint, check-jsonschema, kyverno,
promtool and chainsaw. Local (dev.sh) and CI now gate on the identical toolchain.

- actions/checkout (Node) replaced by a plain-git step with fetch-depth-0 semantics; the
  toolbox has no Node. The job token is not persisted into .git/config.
- The install step is replaced by a check that every ci/versions.env pin equals the image's
  /etc/dev-toolchain-versions, so the pins stay enforced.
- Renovate follows the digest through a `# renovate-digest:` marker; github-actions manager
  disabled for own images.
- install-tools.sh kept (merge-gate.yaml blueprint + alert-rules-unit-tested check use it).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

ci/gate.sh

@@ -46,8 +46,8 @@
 # WHICH SKIPS BLOCK, AND WHY (per stage — a skip is not one thing):
 #
 #   lint / l0 / render-golden / l1   A skip here means a PINNED tool is absent or a wired
-#     seam went unwired. On a CI runner the pinned toolchain is installed for us by
-#     .gitea/workflows/scripts/install-tools.sh, so a skip there is a DEFECT, not an
+#     seam went unwired. On a CI runner the pinned toolchain is the job image itself (the
+#     CI-built toolbox, .gitea/workflows/gate.yml), so a skip there is a DEFECT, not an
 #     environment: it BLOCKS. On a workstation a missing tool is a real local condition, so
 #     it degrades to UNDETERMINED (still non-zero, still never a pass). See strict_skips()
 #     in ci/lib.sh for how CI is detected and why misdetecting it cannot fail open.
@@ -257,7 +257,7 @@ for s in lint l0 render-golden l1; do
     skip)
       if strict_skips; then
         err "${s}: skipped (unwired seam / missing tool) under STRICT skips, where the pinned"
-        err "  toolchain is installed for us (.gitea/workflows/scripts/install-tools.sh) and every"
+        err "  toolchain is baked into the job image (.gitea/workflows/gate.yml container:) and every"
         err "  seam is on main — so this is a DEFECT, not an environment -> blocks merge"
         summary "- ${s}: **SKIPPED under strict skips** (unwired seam / missing tool) — blocks merge"
         BLOCK=1