feat(registry): add GitHub registry support (#10842)

* feat: add github scheme

* fix

* fix: validate and search

* docs: update docs for GitHub registries

* docs: add changelog

* fix

* chore: update announcement

* docs(skills): update GitHub registry guidance

* fix(registry): reject option-like GitHub refs

* fix(registry): limit search registry discovery

* fix(registry): bound GitHub validation concurrency

* fix(registry): reject whitespace in GitHub refs

* fix(registry): track URL dependency sources

* test(registry): cover local dependency sources