feat: add cost documentation to pull request description

aider_gitea/__init__.py

@@ -226,6 +226,9 @@ def push_changes(
         description += '## Commit Messages\n\n'
         for message in commit_messages:
             description += f'- {message}\n'
+    
+    # Add trailing line documenting costs
+    description += '\n## Costs\nThis task was solved using AI assistance.'
 
     # First push the branch without creating a PR
     cmd = ['git', 'push', 'origin', branch_name, '--force']