Claude Code integration

[ScriptRaccoon]

This PR continues #393 and integrates Claude Code into the project.

• The AI guidelines are updated. AI-generated code and proofs are fine as long as the author understands every detail and takes responsibility for them.
• A script pnpm db:structure <ID> is added that lists the properties of a given structure, grouped by their status (satisfied / unsatisfied / unknown / undecidable). This is particularly useful for AI tools working on proofs. An optional parameter specifies the status, for example pnpm db:structure CMon unknown lists the unknown properties of CMon.
• The GitHub Copilot skills are removed, effectively reverting Add GitHub Copilot skills #388.
• The file CLAUDE.md is added. It describes CatDat in detail and serves as project memory for Claude Code.
• Game changer. A new Claude skill (structure-proofs) is added that repeatedly proves new properties of a given structure. The incremental approach is very much inspired by the "human" approach I have been using for months.
• To test the new skill, it has been applied to the category of finitely generated groups – with great success. This is a separate PR: Add the category of finitely generated groups #396
• Aside: the combinations script has been refactored, the type doesn't need to be passed anymore.

I am not 100% sure yet whether I want to pursue this path of automating the mathematics, but I will experiment with it.