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.

.gitea/workflows/ci.yml

@@ -0,0 +1,26 @@
+name: CI
+
+on:
+  push:
+    branches: ['main']
+  pull_request:
+    branches: ['**']
+
+jobs:
+  test:
+    name: Tests, lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+      - name: Install
+        # Pinned: a newer ruff or shellcheck can turn an untouched tree red.
+        run: pip install pytest==8.4.2 ruff==0.14.0 pyyaml==6.0.2 shellcheck-py==0.11.0.1
+      - name: ruff
+        run: ruff format --check . && ruff check .
+      - name: shellcheck
+        run: shellcheck bin/*.sh image_tools/cis_os.sh
+      - name: pytest
+        run: pytest -q