feat: initial MokoOpenGraph package scaffold

Joomla package (pkg_mokoog) with three sub-extensions:
- com_mokoog: Admin component with tag manager, MVC, database schema
- plg_system_mokoog: OG + Twitter Card meta tag injection via onBeforeCompileHead
- plg_content_mokoog: Per-article and per-menu-item OG fields in editor

Includes CI/CD workflows, issue templates, 12 feature issues, and
full Joomla 4/5 DI container architecture.

Authored-by: Moko Consulting

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

src/packages/com_mokoog/sql/uninstall.mysql.sql

@@ -0,0 +1,5 @@
+--
+-- MokoOpenGraph - Uninstall
+--
+
+DROP TABLE IF EXISTS `#__mokoog_tags`;