fix: use loguru for provider logging

Nanobot uses loguru, not stdlib logging. Switch to loguru so
thinking/usage logs actually appear in container output.

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

nanobot/providers/anthropic_oauth.py

@@ -5,12 +5,10 @@ which require Authorization: Bearer header instead of x-api-key.
 """
 
 import json
-import logging
 from typing import Any
 
 import httpx
-
-logger = logging.getLogger(__name__)
+from loguru import logger
 
 from nanobot.providers.base import LLMProvider, LLMResponse, ToolCallRequest
 from nanobot.providers.oauth_utils import get_auth_headers, get_claude_code_system_prefix