Update _register_memory_tools to import memory_store (the current
function name in memory_system.py) instead of memory_write, which
no longer exists after the tools.py breakup in #1215.
The tool is still registered under the name "memory_write" for
backward compatibility with any callers expecting that tool name.
Fixes#1249
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>