When sync.branch is configured, the main branch's .beads/ directory was
showing as modified after every sync, even though the data was correctly
synced to the sync branch worktree.
This happened because:
1. Export writes to main's .beads/
2. Files are copied to worktree and committed there
3. But main's .beads/ now differs from what's committed on main
Fix: After sync completes, restore .beads/ from HEAD to keep the working
directory clean. The actual beads data lives on the sync branch; the main
branch's .beads/ is just a snapshot that should match what's committed.
9483ab053e