add a skill to make it easier for agentic use of manywidgets

[batpad]

Adds

• a skill folder with instructions to agents on how to use manywidgets in their notebooks
• generates widget references directly from the traitlets definition so the code remains the source of truth for the interface of each widget
• an install-skill command so that users can install the skill locally (by default copies the skill into .claude/skills/ in the current folder - with --user copies into user home folder, and --path to install to a custom location

I think I like this pattern for adding a skill to a library - have it generate based on a single source of truth in the code, and add a simple CLI command for users to install it to their favourite location. @kylebarron might be nice to get your thoughts on something similar for lonboard :)

@wrynearson you're likely to be the first user for this, will be great if you're able to give a quick review, though I may also merge it in and we can iterate.

[kylebarron]

I would love to have something similar for lonboard. If you have any interest, feel free to make a PR to Lonboard!

If you wanted, you could even write a post on the Lonboard blog about how to use the new Lonboard AI skills you use 😍

[batpad]

I don't see any major harm in going ahead and merging this and iterating if needed. Merging!