[Feature] Export code to GitHub Gist

[kom50]

Summary

One-click publish code to GitHub Gist.

Requirements

• "Export to Gist" button
• GitHub OAuth or Personal Access Token input
• Create public or secret gist
• Show Gist URL after creation
• Copy Gist URL to clipboard

Technical Notes

• GitHub Gist API: POST /gists
• Requires OAuth token with gist scope

Acceptance Criteria

• User can authenticate with GitHub
• Gist is created successfully
• User receives link to created Gist