fix(lint): resolve all golangci-lint errors

- Mark unused ctx parameter with underscore in getRepoRootForWorktree
- Replace exec.Command("test", "-d") with os.Stat for directory check
- Handle file.Close() errors properly in compact.go and migrate_tombstones.go
- Explicitly ignore cleanup errors with _ assignment

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/sync.go

@@ -836,7 +836,7 @@ func gitHasChanges(ctx context.Context, filePath string) (bool, error) {
 
 // getRepoRootForWorktree returns the main repository root for running git commands
 // This is always the main repository root, never the worktree root
-func getRepoRootForWorktree(ctx context.Context) string {
+func getRepoRootForWorktree(_ context.Context) string {
 	repoRoot, err := git.GetMainRepoRoot()
 	if err != nil {
 		// Fallback to current directory if GetMainRepoRoot fails