refactor: move Socials approval to Data Machine PendingActionStore

[chubes4]

Problem

Studio’s Socials review flow still creates fake pending WordPress posts with _studio_social_* metadata and publishes on post-status transitions. This duplicates action storage, review state, resolution, and cleanup behavior already owned by Data Machine.

Current legacy paths:

• src/blocks/studio/tabs/socials/publish/index.ts::submitForReview()
• inc/social-drafts.php

Goal

Represent each Socials review request as a Data Machine pending action and remove the post-backed approval lifecycle.

Upstream readiness

The earlier observer-dispatch and signed-resolution prerequisites have landed. Re-read the current PendingActionStore and Data Machine Socials contracts before implementation; do not rely on historical API names in old comments.

Migration sequence

1. Document the current pending-action create/read/resolve contract.
2. Submit caption, platform, media, requester, and provenance as a bounded Socials action payload.
3. Resolve approval through the existing Data Machine Socials publish handler.
4. Migrate or explicitly dispose of existing _studio_social_* pending posts.
5. Remove transition-post-status publishing and obsolete meta registration.
6. Preserve direct Publish Now behavior.

Acceptance criteria

• Submit for Review creates no WordPress post.
• Pending actions are idempotent and auditable.
• Approval and rejection re-check reviewer authorization.
• Publish outcomes and platform errors are stored on the action/receipt path.
• Existing pending records have a documented one-time migration or cleanup result.
• inc/social-drafts.php legacy storage and transition behavior are removed.
• Tests cover create, duplicate request, approve, reject, unauthorized resolve, and publish failure.