fix: add from __future__ import annotations to fix Python <3.11 compat

These two files from upstream use PEP 604 union syntax (str | None)
without the future annotations import. While the project requires
Python >=3.11, this makes local testing possible on 3.9/3.10.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nanobot/agent/skills.py

@@ -1,5 +1,7 @@
 """Skills loader for agent capabilities."""
 
+from __future__ import annotations
+
 import json
 import os
 import re

nanobot/utils/helpers.py

@@ -1,5 +1,7 @@
 """Utility functions for nanobot."""
 
+from __future__ import annotations
+
 import re
 from datetime import datetime
 from pathlib import Path