Add Friends abilities for AI Assistant

[akirk]

Summary

• Add a Friends WordPress Abilities API integration for AI Assistant
• Expose abilities for subscriptions, feed discovery, cached feed items, bulk refresh, and single-feed refresh
• Add callback-focused tests and an unreleased changelog entry

Testing

• composer lint
• composer check-cs
• git diff --check
• PHPUnit not run locally: /tmp/wordpress-tests-lib/includes/functions.php is missing

[github-actions]

Test this PR in WordPress Playground

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Launch WordPress Playground

This will install and activate the plugin with the changes from this PR.