If they live in a subdirectory, you'll just have to specify it.
% them automatically.
%
% The sole argument is the "name" of the listing, which ultimately
-% just identifies a file named "sage_listings/<name>.listing".
+% just identifies a file named "<name>.listing".
\newtcbinputlisting{\sagelisting}[2][]{%
colback=codebg,%
coltext=codefg,%
left=2em,%
listing only,%
listing options={language=sage,style=sage},%
- listing file=sage_listings/#2.listing,%
+ listing file=#2.listing,%
#1}
\fi