Make visual-explainer directly installable via pi install

[vedang]

This PR makes the following changes to visual-explainer:

1. Adds a pi manifest to package.json, so that pi can recognize it as an extension
2. Replaces {{skill_dir}} with relative-path wording (because Pi does not understand this variable)

I've also updated the README with relevant information and tested that the change works locally.

[nicobailon]

Thanks @vedang for the Pi-install proposal. Current main has a broader Pi package manifest, documentation, and a native visual_explainer tool. The share-script portion no longer applies because sharing was removed. Closing this as superseded, with credit preserved for the original Pi-install direction.