fix(warp): fix flow for multi arch

[gondoi]

No description provided.

[cursor]

PR Summary

Medium Risk
Changes the Warp image publishing path in the reusable workflow, affecting how multi-arch images are built and tagged; mistakes could break releases or produce incorrect manifests/digests.

Overview
Fixes Warp multi-arch builds in the reusable docker-build-push workflow by splitting comma-separated platforms into per-architecture digest builds and then merging them into a multi-arch manifest via docker buildx imagetools, avoiding Warp scheduling issues that caused arm64 exec-format failures.

Adds new Warp jobs (warp-multi-plan/warp-multi-build/warp-multi-merge) alongside a simplified warp-single path for single-platform builds, and updates workflow outputs to reference the new job names. Documentation in the composite action and workflow is updated to steer callers toward the workflow for multi-arch Warp builds.

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