feat(sync): add resolveBranchHead to fetch branch tip SHA

packages/backend/tests/unit/github-head.test.ts

@@ -0,0 +1,24 @@
+import type { Octokit } from '@octokit/rest';
+import { describe, expect, it, vi } from 'vitest';
+
+import { resolveBranchHead } from '../../convex/githubClient';
+
+describe('resolveBranchHead', () => {
+  it('returns the branch tip sha from the branch ref', async () => {
+    const getBranch = vi.fn(async () => ({
+      data: { commit: { sha: 'deadbeef' } },
+    }));
+    const octokit = {
+      rest: { repos: { getBranch } },
+    } as unknown as Octokit;
+
+    const result = await resolveBranchHead(octokit, 'o', 'r', 'main');
+
+    expect(result.sha).toBe('deadbeef');
+    expect(getBranch).toHaveBeenCalledWith({
+      owner: 'o',
+      repo: 'r',
+      branch: 'main',
+    });
+  });
+});