Installed skills now work outside the DataPressr repo

After npx skills add datasets/datapressr, init now scaffolds a dataset with its conventions file and validator in an empty folder, and validate runs the real checks instead of improvising them. The skills carry what they need and link to everything else by full URL, and capture files ideas in your own project (Beads, its issues or a local INBOX.md) rather than in DataPressr's inbox.