Add working_directory/2 to library(files) imports

The find_project_root/1 predicate uses working_directory/2, which is
exported from library(files), not library(os). This fixes the CI
failure that occurred after PR bakaq#45 changed imports to be explicit.

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

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

Author: jjtolton

bakage.pl

@@ -27,7 +27,7 @@
 :- use_module(library(os), [argv/1, setenv/2, shell/1, unsetenv/1, getenv/2]).
 :- use_module(library(files), [
     directory_exists/1, make_directory_path/1, directory_files/2, file_exists/1,
-    delete_file/1, delete_directory/1
+    delete_file/1, delete_directory/1, working_directory/2
 ]).
 :- use_module(library(dcgs), [phrase/2, phrase/3, ... //0]). % 13211-3
 :- use_module(library(charsio), [write_term_to_chars/3]).