So yeah, I agree that it's all just documentation. I know there's been some evidence shown that skills work better, but my feeling is that in the long run it'll fall to the wayside, like prompt engineering, for a couple of reasons. First, many skills will just become unnecessary - models will be able to make slide decks or do frontend design without specific skills (Gemini's already excellent at design without anything beyond the base model, imho). Second, increased context windows and overall intelligence will obviate the need for the specific skills paradigm. You can just throw all the stuff you want Claude to know in your claude.md and call it a day.
I'd like a user writeable, LLM readable, LLM non-writable character/sequence. That would make it a lot easier to know at a glance that a command/file/directory/username/password wasn't going to end up in context and being used by a rogue agent.
It wouldn't be fool proof, since it could probably find some other tool out there to generate it (eg write-me some unicode python), but it's something I haven't heard of that sounds useful. If it could be made fool/tool proof (fools and tools are so resourceful) that would be even better.
To overly programmer-brain it, a slash command is just a skill with a null frontmatter. This means that it doesn't participate in progressive disclosure, aka Claude won't consider invoking it automatically.