The github provider registered context-files (.github/copilot-instructions.md)
and instructions (.github/instructions/*.instructions.md), but no skills
capability — so .github/skills/<name>/SKILL.md, the layout GitHub documents
for Copilot Agent Skills, was silently never discovered. The skill://
URL resolved to 'Available: none' and nothing surfaced in the system prompt.
Register a skill capability on the github provider (priority 30, project-only)
pointing at .github/skills/ and reuse scanSkillsFromDir with
requireDescription: true to match the Agent Skills spec and the sibling
native/omp-plugins providers. Pin the wiring with a discovery test that
loads the skills capability scoped to the github provider against a temp
cwd containing a SKILL.md, and a negative case that drops a skill missing
a description.
Fixes#1906
roboomp