chore: remove unused ABC import from config_source.py

ConfigSource uses Protocol, not ABC. Removed unused import.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/central/config_source.py

@@ -5,7 +5,6 @@ either TOML files or the database-backed config store.
 """
 
 import logging
-from abc import ABC, abstractmethod
 from collections.abc import Awaitable, Callable
 from pathlib import Path
 from typing import Any, Protocol, runtime_checkable