feat: judge, sign and verify netOrk's images in one place (NetOrk/netork#1390)

The CIS gate, the SBOM, the cosign signature and the matcher's verdict lived
in NetOrk/base-os, the head of the chain. Every link below it -- base-python,
engine, ui, satellite, the service images -- needs the same, so the tools
move here, and every image repository pins them by commit.

- image_tools: cis.py, cis_os.sh, sbom.py and verify.py from base-os. The
  image's own repository holds its cis.toml and packages.allow; both default
  to the working directory.
- bin/sign.sh signs whatever platforms a reference holds: base-os's index of
  amd64 and arm64 (EXPECT_PLATFORMS) or a dev image's single manifest
  (image_tools/platforms.py).
- bin/verify-base.sh checks every FROM image of a Dockerfile against
  cosign.pub before a build; one without a digest is refused
  (image_tools/dockerfile.py).
- bin/install-cosign.sh installs cosign v3.1.3 for amd64 and arm64 runners,
  checked against the release checksums.
- actions/verify-base, judge, sign wrap them for the workflows.
- cosign.pub is the public half of the COSIGN_PRIVATE_KEY_B64 org secret,
  the key rotated on 2026-10-22.

bin/verify-base.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+# verify-base.sh <Dockerfile>...: every image the files build on is signed with
+# netOrk's key, checked before the build (NetOrk/netork#1390).
+#
+# The images are the FROM lines' references (image_tools/dockerfile.py): stages
+# of the same file and scratch are none, and one without a digest is refused.
+set -euo pipefail
+tools=$(cd "$(dirname "$0")/.." && pwd)
+[ $# -gt 0 ] || { echo "usage: verify-base.sh <Dockerfile>..." >&2; exit 2; }
+files=()
+for f in "$@"; do files+=("$(cd "$(dirname "$f")" && pwd)/$(basename "$f")"); done
+mapfile -t bases < <(cd "$tools" && python3 -m image_tools.dockerfile "${files[@]}")
+[ ${#bases[@]} -gt 0 ] || { echo "no image to verify in $*" >&2; exit 1; }
+"$tools/bin/verify-signature.sh" "${bases[@]}"