Add workflow to publish unstable distribution assets

[KaiKitJeffreyChan]

Summary

Adds a workflow that refreshes the rolling unstable prerelease when taxonomy data or dev changes on main.

• Generates distribution files for all locales.
• Reuses the staging command from Publish stable release distribution assets #957.
• Validates asset counts and gzip integrity before modifying the release.
• Creates the prerelease when absent and replaces assets with --clobber.
• Reconciles published assets against the staged set after upload: deletes orphaned assets and fails if any staged asset is missing from the release.
• Serializes runs to prevent overlapping publications.
• Supports workflow_dispatch for manual recovery re-runs, guarded to refuse non-main refs.
• Posts a Slack message via incoming webhook when the publish job fails, so failures don't rely on the push actor's email (~83% of data/** commits are the translation bot).

Stacked on #967, which adds return reason localizations to the validator this workflow relies on for all-or-nothing generation.

Closes https://github.com/shop/issues-taxonomy/issues/542.

Known gaps (acknowledged)

• --clobber alone can't remove or verify assets — it only adds/replaces name-matched assets. The reconcile step closes this. Asset replacement is still non-atomic (GitHub has no transactional multi-asset swap), so a brief mixed window exists mid-run; every green run converges to the exact staged set.
• Translation lag turns runs red — English data/** changes land days before bot translations; the localization validator fails the publish during that window. Accepted: fail-loud is intended, assets stay at last-good, and the run self-heals once translations merge. Will also request that data changes arrive alongside localizations.
• Missing localized descriptions silently fall back to English — the validator gates name only; a translated entry lacking a description ships English text with a green run. Accepted as fallback-by-design; verified zero occurrences across all ~280k localized attribute/return-reason entries today.
• CI generates dist in English only — locale-specific generation bugs (the Fix AlphanumericSorter crash on mixed sequential-text values #942 class) surface first in this workflow, not CI. Accepted: this workflow is the all-locale canary; failures alert via Slack and recover via workflow_dispatch re-run.

[KaiKitJeffreyChan]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add workflow to publish unstable distribution assets #960 👈 (View in Graphite)
• Validate return reason localizations #967
• main

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[hadijafar]

looks good