Add ko image build + push to ATCR in CI
After tests pass, the CI pipeline now builds an OCI image with ko and
pushes it to atcr.io/guid.foo/wicket. Tags each image as :latest and
with a timestamp+sha tag (e.g. 20260304201500-abc1234) so FluxCD's
image automation can pick up new builds in order.
Uses chainguard/static as the base image via .ko.yaml — same idea as
our scratch Dockerfile but with tzdata and CA certs baked in.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>