fix: support syncing to a brand-new empty repository - #4

Summary

Fix synchronization when the target GitHub repository is brand new and has no commits yet.

Problem

getTree() calls the GitHub Trees API for the configured branch. For a newly created empty repository, GitHub responds with:

409 Git Repository is empty.

The plugin currently treats this as a fatal error, so the initial sync cannot proceed far enough to create the first commit.

Fix

Detect this specific empty-repository response and return an empty tree:

{ sha: "", url: "", tree: [], truncated: false }

This lets the existing sync flow continue and create the first files/commit through the Contents API.

Other 409 responses are still treated as errors.

Testing

Tested with a brand-new empty GitHub repository; the initial synchronization can now proceed successfully.

This PR is intentionally scoped to the empty-repository fix only.