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.

tests/test_actions.py

@@ -0,0 +1,38 @@
+"""The composite actions every image repository of NetOrk uses (NetOrk/netork#1390).
+
+    uses: https://git.netork.io/NetOrk/image-tools/actions/<name>@<commit>
+
+They only call what this repository holds, so a pinned commit fixes what runs.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+import pytest
+import yaml
+
+ROOT = Path(__file__).resolve().parents[1]
+ACTIONS = sorted((ROOT / "actions").glob("*/action.yml"))
+
+
+def test_the_three_actions():
+    assert [a.parent.name for a in ACTIONS] == ["judge", "sign", "verify-base"]
+
+
+@pytest.mark.parametrize("action", ACTIONS, ids=lambda a: a.parent.name)
+def test_an_action_runs_only_what_this_repository_holds(action):
+    spec = yaml.safe_load(action.read_text())
+
+    assert spec["runs"]["using"] == "composite"
+    for step in spec["runs"]["steps"]:
+        for script in re.findall(r"github\.action_path }}/\.\./\.\./(bin/[\w.-]+)", step["run"]):
+            assert (ROOT / script).is_file(), script
+        assert "${{ inputs." not in step["run"], "inputs reach a script through env"
+
+
+def test_signing_needs_its_secrets_from_the_caller():
+    text = (ROOT / "actions/sign/action.yml").read_text()
+
+    assert "COSIGN_PRIVATE_KEY_B64" in text and "secrets." not in text