feat(github): expand github workflows and actions

[gondoi]

Expand GitHub action functionality and use them in shareable workflows.

[cursor]

PR Summary

Medium Risk
Introduces new reusable build/publish pipelines (Docker multi-arch digest merge, ECR OIDC, Helm OCI) and rewires legacy workflows to them; misconfiguration could break image/chart publishing or notifications.

Overview
Adds a new OCI publishing toolkit under .github/ with composites for docker-build-push (ECR OIDC/private+public or registry login; Buildx/QEMU or Warp), helm-publish-oci (Helm lint + OCI push with registry token or ECR OIDC), and a lightweight slack-notify-failure step.

Introduces reusable workflow_call wrappers docker-build-push.yml (native per-platform builds that push by digest and then merge manifests, or Warp direct push) and helm-publish-oci.yml (single-job chart publish with required permissions).

Updates legacy publish-docker-image.yml to forward to the new Docker workflow (splitting comma-separated images into a matrix) and adjusts Slack failure notifications in publish-crate.yml; refreshes top-level docs and fixes the setups/docker README header.

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[cursor]

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