fix(lint): address errcheck and De Morgan's law violations in doctor/fix

- Handle file.Close() errors in deletions.go and deletions_test.go
- Simplify boolean logic to apply De Morgan's law in common.go
- All golangci-lint checks now pass

cmd/bd/doctor/fix/common.go

@@ -86,5 +86,5 @@ func isWithinWorkspace(root, candidate string) bool {
 	if err != nil {
 		return false
 	}
-	return rel == "." || !(rel == ".." || strings.HasPrefix(rel, ".."+string(os.PathSeparator)))
+	return rel == "." || (rel != ".." && !strings.HasPrefix(rel, ".."+string(os.PathSeparator)))
 }