fix(sync): add --sparse flag to git add for worktree compatibility (#1143)

Fixes #1076.

When git worktrees have sparse-checkout enabled, git add operations can
fail with 'outside of sparse-checkout cone' errors. Adding the --sparse
flag allows git to stage files outside the sparse-checkout cone.

Changes:
- Add --sparse flag to git add in daemon_sync_branch.go (line 165)
- Add --sparse flag to git add in migrate_sync.go (line 343)
- Add --sparse flag to git add in sync_branch.go (line 308)
- Add --sparse flag to git add in worktree.go (line 732)

All changes include comments referencing #1076 for traceability.

cmd/bd/migrate_sync.go

@@ -339,7 +339,8 @@ func commitInitialSyncState(ctx context.Context, worktreePath, jsonlRelPath stri
 	beadsRelDir := filepath.Dir(jsonlRelPath)
 
 	// Stage all beads files
-	addCmd := exec.CommandContext(ctx, "git", "-C", worktreePath, "add", beadsRelDir)
+	// Use --sparse to work correctly with sparse-checkout enabled worktrees (fixes #1076)
+	addCmd := exec.CommandContext(ctx, "git", "-C", worktreePath, "add", "--sparse", beadsRelDir)
 	if err := addCmd.Run(); err != nil {
 		return fmt.Errorf("git add failed: %w", err)
 	}