Install from GitHub and skills.sh

SkillHub learned to consume the wider ecosystem, not just your own registries:

• Install straight from a GitHub repo (gh:owner/repo) or the skills.sh index (sh:name).
• skillhub import external skills into a registry with recorded provenance and upstream-drift detection, so you can tell when a source has moved on.
• Run a skill once, without installing, via skillhub use.
• skillhub search across your registries, skills.sh, and GitHub at once.