feat: add GitHub user auto-detection and auto-create sync repo

[iHildy]

Add getAuthenticatedUser and repoExists helpers and a default repo name my-opencode-config. Update init flow to auto-create a private repo when missing and refresh README and command docs.

[gemini-code-assist]

Summary of Changes

Hello @iHildy, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the user experience for setting up opencode-sync by automating key configuration steps. It introduces intelligent detection of the user's GitHub identity and the ability to automatically provision a dedicated private repository for synchronization, making the initial setup much more intuitive and less prone to manual errors. The changes are reflected in updated documentation, guiding users through the new streamlined process.

Highlights

• Automated GitHub Username Detection: The /sync-init command now automatically detects the authenticated GitHub username using the gh CLI tool, streamlining the setup process.
• Automatic Repository Creation: If the specified or default sync repository does not exist, /sync-init will now automatically create a private GitHub repository for the user.
• Default Sync Repository Name: A default repository name, my-opencode-config, is now used when no custom repository name is provided during initialization.
• Updated Documentation: The README.md and src/command/sync-init.md files have been updated to reflect the new, simplified /sync-init workflow and its capabilities.

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[gemini-code-assist]

Code Review

This pull request introduces a great enhancement to the user experience by auto-detecting the GitHub user and auto-creating the sync repository. The code is generally well-structured, and the documentation updates in the README are clear and helpful. However, I've identified a couple of high-severity issues in the logic for parsing repository options in src/sync/service.ts. These issues could lead to user input being silently ignored or mis-handled, resulting in a confusing user experience. Addressing these will make the new feature much more robust.