Summary

Extracts the friend_tag taxonomy logic into a dedicated Friend_Tag class (includes/class-friend-tag.php), replacing scattered inline usage of the taxonomy throughout the codebase.

Changes

• New Friend_Tag class with static methods for all tag operations: register(), has_tag(), has_mention(), add_tag(), add_tags(), mention_tag(), remove_tag(), cleanup_orphaned(), etc.
• Friends class now delegates to Friend_Tag::register() and Friend_Tag::cleanup_orphaned() instead of inlining the taxonomy registration and cleanup logic. Friends::TAG_TAXONOMY references Friend_Tag::TAXONOMY.
• Feed class uses Friend_Tag::add_tags() instead of calling wp_set_post_terms() directly for both friend_tags and friend_mention_tags.
• ActivityPub parser uses Friend_Tag::mention_tag() to build mention slugs and Friend_Tag::has_mention() to check for mentions before queuing reply-to-comment conversion.
• Migration class replaces inline wp_get_post_terms() + prefix-matching loops with Friend_Tag::has_mention() and uses Friend_Tag::mention_tag() for building mention slugs.
• Enable Mastodon Apps integration uses Friend_Tag::mention_tag() instead of hardcoding the mention- prefix.

Motivation

The mention- prefix and friend_tag taxonomy name were hardcoded in many places. This consolidates them behind constants (Friend_Tag::TAXONOMY, Friend_Tag::MENTION_PREFIX) and a single class, making the tag system easier to maintain and less error-prone.