fix: handle git config lock errors gracefully

[Kcstring]

Summary

• catch OSError from repo.config_writer() in setup_git()
• warn the user how to set user.name and user.email manually instead of crashing when .git/config.lock exists
• add a regression test for the locked config path

Testing

• python3 -m pytest tests/basic/test_main.py -k "setup_git or config_writer_lock"
• python3 -m ruff check aider/main.py tests/basic/test_main.py

Fixes #5060