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.

pyproject.toml

@@ -0,0 +1,23 @@
+[project]
+name = "image-tools"
+version = "0.1.0"
+description = "Judge, sign and verify netOrk container images"
+requires-python = ">=3.11"
+license = "MIT"
+
+[build-system]
+requires = ["setuptools>=69"]
+build-backend = "setuptools.build_meta"
+
+[tool.setuptools]
+packages = ["image_tools"]
+
+[tool.setuptools.package-data]
+image_tools = ["cis_os.sh"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["."]
+
+[tool.ruff]
+line-length = 100