When sync.branch is configured, the sync command:
1. Exports changes to JSONL
2. Commits to sync branch via worktree
3. Pulls from sync branch
4. Restores .beads/ from HEAD to keep working directory clean
But PersistentPostRun's flushManager.Shutdown() was re-exporting to JSONL,
undoing the restore and leaving modified files.
Fix: Set skipFinalFlush flag when sync.branch mode completes successfully.
This prevents the final export in PersistentPostRun.
Also: Skip push to main branch when sync.branch is configured - all
pushes should go through the sync branch worktree.
8ef3d81108