GitHubPusher: read initial blob_sha from base_branch

Without ref:, the API defaults to the repo's default branch,
which differs from base_branch when the conference specifies a
custom branch. This caused a guaranteed conflict on the first
update_contents attempt, wasting a retry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sorah

app/jobs/generate_sponsors_yaml_file_job.rb

@@ -188,7 +188,7 @@ def reset_branch
 
     def commit_content
       begin
-        blob_sha = octokit.contents(@repo.name, path: @filepath)[:sha]
+        blob_sha = octokit.contents(@repo.name, path: @filepath, ref: base_branch)[:sha]
       rescue Octokit::NotFound
         blob_sha = nil
       end