feat: GitHub Action - jedyna powierzchnia odkrywania, na ktorej nas nie ma - #222

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Read against GitHub's own documentation rather than recalled. Actions in the
Marketplace are not reviewed and appear immediately, provided the repository is
public, holds a single action.yml at its root, and the name is not already
taken or reserved. All three hold. The listing itself needs one thing a person
must do once: draft a release with "Publish this Action to the GitHub
Marketplace" ticked, which requires accepting the Marketplace Developer
Agreement the first time.

Why this and not another page: the Marketplace is browsed by category and has
its own search, so it reaches people who will never see this repository. Topics
are maxed at twenty, the social preview is set, releases are running, and the
directory submissions are open. This is the surface that was simply empty.

The action is not a wrapper for its own sake. It adds the thing that makes a
tool worth putting in a pipeline rather than running once: it fails the job
when the mail server's certificate is inside a window you choose. Nobody
watches a mail certificate. It expires on a Sunday and the first report is a
user saying scanning stopped working. cert-expiry-days turns that into a build
failure two weeks earlier.

fail-on-error exists because the other use is a scheduled canary, and a canary
that pages somebody at 3am for a relay that is not theirs is a canary that gets
deleted.

No install step and no npm. The tool has no dependencies, so the action runs
the file that ships beside it, which also means it cannot break because a
registry was slow or a tag moved.

CI runs the action from this repository on every change to the code and asserts
that its outputs are usable - ok is a boolean, the JSON carries every port
asked for. A Marketplace listing that does not run is worse than no listing: it
is a public page that fails for whoever tries it first.