Summary:

- Added a URL mode to Skill Creator for importing GitHub Markdown files.
- Added clipboard prefill and command palette entry for creating a skill
from URL.
- Reused skill frontmatter parsing so imported skills prefill metadata
and content.
- Added Command Palette command to add skills via URL.

Closes AI-301
Release Notes:

- Added Skill Creator support for importing skills from GitHub Markdown
URLs

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>