zizmor audits the workflows themselves for what makes a workflow
unsafe, and uploads what it finds to code scanning. Its first run here
found one thing, 4 times: a checkout that leaves the job's token in
the checked-out repository, where any later step can read it. No
workflow pushes or needs the token after the checkout, so every checkout
now sets persist-credentials to false, and zizmor reports nothing.

OpenSSF Scorecard grades how the repository is kept: branch protection,
pinned dependencies, token permissions and the like. It runs on the
default branch and weekly, publishes its score and uploads its findings to
code scanning.

Dependency Review fails a pull request that adds or changes a dependency
with a known vulnerability of high severity or worse. Dependabot watches
what is already there; this holds what comes in.

Every new action is pinned to a commit, each job names the permissions
it needs, and no new workflow has a default permission.