enhance: split available experiments

[sapayth]

What?

Closes #218

Organizes AI experiments into categories. Editor features (content generation, alt text) and Admin features (exploration tools) with separate sections in the settings page.

Why?

Update the AI Experiments settings screen to group available experiments into two clear sections: Editor Experiments and Admin Experiments. This improves clarity for users by distinguishing features that appear in the editor from tools intended for site administration, exploration, or developer experience.

How?

• Added Experiment_Area class with EDITOR and ADMIN constants
• Added get_category() method to the Experiment interface
• All experiments must now define a category in their metadata
• Experiments are grouped in the settings UI by category instead of a single list

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Used for: Initial implementation suggestions and refactoring; all code was reviewed, tested, and edited by me.
All changes have been reviewed and tested manually. Code is licensed under GPLv2 or later.

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: sapayth <sapayth@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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