style: align comments with the project conventions

Apply a codebase-wide comment audit against the comment conventions:
split banned semicolons and multi-sentence lines into one sentence per
line, cut cross-file and history narration, trim file-top headers to a
single purpose line, and drop verbosity that did not earn its place.
Prose docs (CLAUDE.md, install.md) get the same one-sentence-per-line
and no-semicolon treatment.

modules/desktop/userdirs.nix

@@ -9,8 +9,7 @@ in
   options.modules.desktop.userdirs.enable = lib.mkEnableOption "XDG user directories";
 
   config = lib.mkIf cfg.enable {
-    # enable writes ~/.config/user-dirs.dirs from the option defaults, which
-    # xdg-user-dir then reads.
+    # Write ~/.config/user-dirs.dirs from the option defaults.
     home-manager.users.${user}.xdg.userDirs.enable = true;
   };
 }