fix(shared): discover global rules at ~/Cline/Rules (#13614)

The VS Code Rules tab resolves the Documents folder via
'xdg-user-dir DOCUMENTS', which prints bare $HOME when no user-dirs
config exists (WSL/headless installs), so it reads and writes global
rules at ~/Cline/Rules. The SDK's rule search paths only covered
~/Documents/Cline/Rules, so those rules never reached the system prompt.
Add the missing path to the search list.

Fixes #13542

Co-authored-by: Saoud Rizwan <saoudrizwan@users.noreply.github.com>