Add WordPress Abilities API Support

[JUVOJustin]

Summary

• Added WordPress Abilities API integration with interface definitions and Loader support
• Includes comprehensive documentation for implementing abilities and categories
• Updates project documentation and instruction files for consistency

Changes

New Features

• Added Ability_Interface (src/Abilities/Ability_Interface.php) - Defines contract for abilities with schemas, permissions, and execution callbacks
• Added Ability_Category_Interface (src/Abilities/Ability_Category_Interface.php) - Defines contract for organizing abilities into categories
• Enhanced Loader class (src/Loader.php:124-321) - Added ability registration with automatic category collection
• Added comprehensive documentation (docs/abilities.md) - Full usage examples and reference

Documentation Updates

• Updated AGENTS.md - Added Abilities API section with implementation guide
• Updated README.md - Updated structure diagram to include Abilities/ directory
• Renamed .opencode/command/upsert-translations.md → translations-upsert.md - Fixed command naming inconsistency
• Updated instruction files for various upsert commands - Fixed path references and clarified documentation

Other Changes

• Updated demo-plugin.php - Bump WordPress requirements to 6.9 (required for Abilities API)
• Consistency improvements across documentation files

Key Implementation Details

The Abilities API integration provides:

• Consistent structure via Ability_Interface enforcing input/output schemas and permissions
• Automatic category registration when abilities reference categories
• Type-safe implementation with PHPStan annotations
• No overhead when disabled - silently skips on WordPress < 6.9 or when no abilities registered