Initial plugin - WP Scheduled Posts Page v1.0.0

Splits the WordPress Posts admin into two views:
- Default Posts page shows only published/draft/pending/private posts
- New "Scheduled Posts" submenu shows future posts ordered by
  scheduled date ascending (soonest first) with countdown display

Refs: Redmine #2409, #2410, #2411

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
