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.

actions/verify-base/action.yml

@@ -0,0 +1,20 @@
+name: Verify the base images
+description: >-
+  Every image the Dockerfiles build on is signed with netOrk's key, checked
+  before the build (NetOrk/netork#1390, bin/verify-base.sh).
+inputs:
+  dockerfiles:
+    description: The Dockerfiles, separated by spaces, relative to the workspace
+    required: true
+runs:
+  using: composite
+  steps:
+    - name: Install cosign
+      shell: bash
+      run: bash "${{ github.action_path }}/../../bin/install-cosign.sh"
+    - name: Verify
+      shell: bash
+      env:
+        DOCKERFILES: ${{ inputs.dockerfiles }}
+      # shellcheck disable=SC2086 # the list splits on purpose
+      run: bash "${{ github.action_path }}/../../bin/verify-base.sh" $DOCKERFILES