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-gh-extensions.sh

@@ -51,8 +51,8 @@ install_extensions()
 # Install all GitHub CLI extensions
 main()
 {
-  install_extensions
-  msgr run_done "Done"
+  install_extensions \
+    && msgr run_done "Done"
   return 0
 }