Add GitHub Copilot App and CLI support

[ShawnXxy]

Summary

• add GitHub Copilot App and CLI as a supported Agent Skills harness
• document project and personal installation paths and commands
• align skill frontmatter and URL-fetch wording with portable Agent Skills conventions
• expose GitHub Copilot in the showcase install panel

Verification

• installed the local skill with npx skills add . --skill hallmark --agent github-copilot --copy --yes
• confirmed discovery with copilot skill list
• ran /hallmark audit site/_tests/verbs/audit/input.html through Copilot CLI and confirmed it made no edits
• ran compatibility assertions, node --check site/js/main.js, npm pack --dry-run --json, and git diff --check

[vercel]

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