feat(skills): sync a skill draft from GitHub

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Summary

• Skill editors can connect their own GitHub account, save a repository source, and sync that revision into the draft.
• The GitHub source section stays collapsed by default and sits directly under reference docs and asset files.
• OAuth app credentials are saved per user in system settings. The callback page finishes the connection and returns to the skill.

Test plan

• Open an editable skill: GitHub source is collapsed and below reference docs and asset files
• Expand it, save a repository, branch or tag, and optional subdirectory
• Connect GitHub from the section, then sync; the draft updates
• Edit the draft and sync again; the overwrite confirmation appears
• A managed skill does not show the GitHub section

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