Support installation with gh skill

[mavam]

Hi! I tried installing markit with GitHub CLI's new gh skill command, but it does not currently discover the skill:

gh skill install Michaelliv/markit markit
# no skills found in Michaelliv/markit

gh skill discovers skills using the Agent Skills spec conventions, especially skills/*/SKILL.md (or a nested .../skills/*/SKILL.md). This repo currently has SKILL.md at the repository root, which works with some tools but not with gh skill discovery.

Suggested fix: move or copy the skill into a standard directory, for example:

skills/markit/SKILL.md

That should make this work:

gh skill install Michaelliv/markit markit

References:

• https://cli.github.com/manual/gh_skill_install
• https://agentskills.io/specification