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

packages/backend/convex/githubClient.ts

@@ -88,6 +88,16 @@ export const getRepository = async (
   return result.data;
 };
 
+export const resolveBranchHead = async (
+  octokit: Octokit,
+  owner: string,
+  repo: string,
+  branch: string,
+): Promise<{ sha: string }> => {
+  const result = await octokit.rest.repos.getBranch({ owner, repo, branch });
+  return { sha: result.data.commit.sha };
+};
+
 const toMillis = (value?: string | null) =>
   value ? new Date(value).getTime() : undefined;