feat(comment): add guest/anonymous comments support

[capy-ai]

This PR adds support for guest/anonymous comments by introducing guest_name and guest_email columns to the comments table, updating the Comment model with conditional validations and helper methods, and providing both new and upgrade migration paths.

• Updated migration templates at lib/generators/acts_as_commentable_with_threading_migration/templates/migration.rb and lib/generators/acts_as_commentable_upgrade_migration/templates/migration.rb to add guest_name and guest_email columns, and make user_id nullable
• Modified Comment model templates in both generators to validate either user or guest_name, validate guest email format, and add commenter_name, guest_comment?, and user_required? helper methods
• Extended Comment.build_from to accept optional guest_name and guest_email while maintaining backward compatibility with the existing user_id parameter
• Added new generator lib/generators/acts_as_commentable_guest_migration for upgrading existing installations to support guest comments
• Updated test schema at spec/db/schema.rb and added guest comment tests at spec/comment_spec.rb
• Documented guest comment usage in README.md

Guest comments require a guest_name when no user is present, email is optional but validated when provided, and commenter_name returns user.name, guest_name, or "Anonymous".