Show license before downloading skill

[TrevorBurnham]

Agent skills can have a license field. Some of the licenses for popular skills are quite strict. For example, Anthropic's skills are licensed in such a way that using a tool like gh-upskill to download them appears to be a license violation:

ADDITIONAL RESTRICTIONS: Notwithstanding anything in the Agreement to the
contrary, users may not:

• Extract these materials from the Services or retain copies of these
  materials outside the Services
• Reproduce or copy these materials, except for temporary copies created
  automatically during authorized use of the Services

I'd suggest that if a skill has a license, gh-upskill should prompt users to accept the license before downloading:

$ gh upskill anthropics/skills --skill pdf

Please read the license terms for this skill before continuing:

> © 2025 Anthropic, PBC. All rights reserved.
>
> LICENSE: ...

Do you want to proceed? (y/n)