Unify skills/commands/rules etc for smoother/tidier dev experience

As far as I can tell, in a short amount of time, both VSCode and Cursor will support the generic Agent Skills standard. At the moment you can do this, but only in the nightly build versions. Once they are available in the main versions, then we should unify.

So I think that will allow:

• removal of .claude/skills/
• removal of .cursor/commands/
• instead put unified stuff into .skills/ or wherever we have to put it

See here https://cursor.com/docs/context/skills and https://code.visualstudio.com/docs/copilot/customization/agent-skills here for more.

That still leaves .cursor/rules. I think this works in the same way as AGENTS.md in that it's just context added to each new agent conversation? So we might be able to do something there. But this is out of scope for this issue.