Support importing skills from external sources (clawhub.ai and skills.sh)
via a new POST /api/skills/import endpoint. The backend auto-detects the
source from the URL, fetches skill metadata and files, and creates the
skill in the workspace. The frontend CreateSkillDialog now has two tabs:
Create (manual) and Import (paste URL with source auto-detection badge).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>