Add AI helper assets for Drupal best practices

[theodorosploumis]

Summary

• add AI-targeted guidance files (AGENTS, rules, commands, Claude Code skills) derived from README sections 2 and 3
• provide drush-based validation scripts by Site building and Theming subsections for Drupal 10+
• document the new AI helper folder and usage

Testing

• Not run (not requested)

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Theming validator uses non-existent Theme entity

The theming script imports Drupal\system\Entity\Theme and calls Theme::loadMultiple(), but Drupal themes are extensions exposed through the theme handler, not config entities. On Drupal 10 this use statement causes drush php:eval to fatally error (Class "Drupal\system\Entity\Theme" not found), so none of the validation logic can run. Please switch to the theme handler (e.g., \Drupal::service('theme_handler')->listInfo()) or another supported API so the script can execute.

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