Fix rm not accepting absolute paths

rm always glued its argument onto the player's current directory, so
typing an absolute path (e.g. sudo rm /var/arrest/handle.sh) built a
garbage path that matched no file and was rejected as "not allowed" -
even with full rm/sudo rights. cd already special-cased a leading '/'
as absolute; rm now does the same.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Game/Service/GameResponseService.php

@@ -240,7 +240,9 @@ class GameResponseService
                 }
 
                 $filename = $messagePart[1];
-                $fullPath = ($pwd === '/' ? '' : $pwd) . '/' . $filename;
+                $fullPath = str_starts_with($filename, '/')
+                    ? $filename
+                    : ($pwd === '/' ? '' : $pwd) . '/' . $filename;
 
                 if(!$this->isAllowedToRemove($fullPath, $player, $sudo))
                     return ['result' => ['You are not allowed to remove this file.']];