fix: guard success messages on command exit status

- Source x-path instead of capturing empty stdout via command substitution
- Validate --dry-run argument in cleanup script, reject unknown flags
- Gate success messages on preceding command's exit status with &&
- Applies to dfm (fmt, reset_nvim, shfmt) and install scripts
  (fonts, gh-extensions, python-packages, shellspec, z)

scripts/install-python-packages.sh

@@ -35,5 +35,5 @@ install_libraries()
   return 0
 }
 
-install_libraries
-msgr yay "Python library installations complete"
+install_libraries \
+  && msgr yay "Python library installations complete"