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]>

.gitea/workflows/README.md

@@ -165,7 +165,8 @@ Cost: the job shares the single act_runner slot with the gate, but the build its
   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)
+    install-tools.sh         installs the pinned tools per stage (from ci/versions.env) — used by the
+                             dormant merge-gate.yaml blueprint only; gate.yml runs in the toolbox image
     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