feat: bump moves here, and verify-base takes images by name

- image_tools/bump.py from base-os, with an action. base-python used it from
  a pinned checkout of base-os, with the bot's token; one place serves both.
  A dependent whose workflow does not exist (netork's ui-nightly.yml, on
  2026-10-22) is named and the others go on; the run fails at the end, as
  for a repository the token cannot see. bump_all holds that loop, tested.
- verify-base takes refs besides Dockerfiles: base-python builds FROM
  ${BASE_OS}, which its nightly sets, and copies CPython from Docker's
  python:3.12-slim, which is not signed with netOrk's key.

README.md

@@ -22,6 +22,7 @@ with its CIS result and SBOM attached (NetOrk/netork#1390).
 | **Signature** (`bin/sign.sh`) | A cosign signature on the reference and on every platform image behind it, with the CIS result and the SBOM as in-toto attestations. Nothing goes to a public transparency log. |
 | **Verification** (`bin/verify-base.sh`, `bin/verify-signature.sh`) | Every `FROM` image of a Dockerfile is pinned by digest and signed with the key in `cosign.pub`. |
 | **Verdict** (`image_tools/verify.py`) | Whether a rebuild improves on the pinned image, by netOrk's matcher (kb-server), for a base's nightly. |
+| **Bump** (`image_tools/bump.py`) | One pull request in every repository of a `dependents.toml` that moves only the pin, and that repository's workflow started. |
 
 ## Using it in a workflow