Revert "STAC-3401 blacklisted processes (#71)" - #75

Ports the publishing half of pre_release_main_agent_image and
pre_release_cluster_agent_image, plus merge_docker_manifest_main_agent
and merge_docker_manifest_cluster_agent, from GitLab to GitHub Actions.
PR #446 landed the build-and-verify halves; this adds the push.

Each image gets a per-arch publish job that needs: the existing image
build job, and a manifest-merge job that assembles the multi-arch tag
from the two single-arch ones. Gated on `github.event_name == 'push'`
alone: the workflow's push filter only carries the release branch, so
the event check is the whole gate, and a workflow_dispatch on an
arbitrary branch cannot publish. This is the same reasoning the
cerberus-notify gate already uses.

Uses the StackVista/image-pipeline composite actions rather than an
open-coded docker push, matching stackstate-process-agent and the
STAC-24837 direction for product repos. Over publish_image.sh that adds
cosign signatures in both v2 and v3 bundle formats, SBOM and max-mode
provenance attestations, canonical SUSE Observability OCI labels, an
entrypoint ELF-architecture check that catches arch-mismatched images
before they are signed, and refusal to overwrite an existing tag.
Neither Dockerfile declares `ARG BASE_IMAGE`, so base-name is passed
explicitly; both final stages are registry.suse.com/bci/bci-micro.

Tag is the 8-character short SHA, matching GitLab's CI_COMMIT_SHORT_SHA.
The `<branch>-<arch>` tag publish_image.sh also pushed is deliberately
dropped: helm-charts-internal pins the agent and cluster-agent images to
an 8-character SHA (currently 9516cb4, the stackstate-7.78.2 HEAD), and
beest receives the tag as AGENT_HASH_UNDER_TEST, so nothing consumes a
branch-name tag. An org-wide code search for
stackstate-k8s-agent:master, :stackstate-7*, :$CI_COMMIT_REF_SLUG and
the cluster-agent equivalents returns no hits, verified against a
positive control so an empty result is not a false negative.

Both publish jobs need id-token: write for keyless cosign signing, and
are added to each workflow's cerberus-notify needs list so a failed
publish on the release branch still reaches Slack.

Requires STAC-25541 (terraform-infra #75): the stackstate+agent robot
behind QUAY_USER has no write grant on stackstate-k8s-agent or
stackstate-k8s-cluster-agent, since GitLab publishes them with the
legacy gitlabci robot instead. Without it these jobs fail exactly as
process-agent did in STAC-25510.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>