Replace hard-coded log path with XDG data directory

[github-actions]

Summary

Replace hard-coded log path with XDG-compliant data directory using get_data_dir(), update tests, and apply style fixes.

Implementation Details

• Imported get_data_dir() in setup_logging() and removed the hard-coded ~/.oai_coding_agent path.
• Updated the docstring to refer to DATA_DIR/agent.log instead of ~/.oai_coding_agent/agent.log.
• Updated tests in tests/test_logger.py and tests/console/test_console.py to import and use get_data_dir() at the top-level.
• Removed duplicate and unused imports (Path) in logger and console modules.
• Sorted and grouped imports in logger.py and console/console.py to satisfy ruff import style rules.

Assumptions & Style Decisions

• Moved test imports of get_data_dir() to the top-level to comply with the existing ruff PLC0415 rule (imports should be at the top). This deviates from the example diff but maintains repository-wide import style.
• No behavior changes beyond switching to the XDG data directory.

Testing

Ran the following checks:

uv run ruff check --fix
uv run ruff format
uv run mypy .
uv run pytest

All checks and tests passed successfully.