Tests for pick_next_game were calling uninstall_other_games and
state.save against real filesystem paths, deleting installed games
and overwriting the user's state.json whenever tests or pre-commit
ran.
- Add conftest.py safety net that redirects STEAMAPPS_PATH,
CONFIG_DIR, STATE_FILE, SNAPSHOT_FILE, CONFIG_FILE, and
HOSTS_FILE to tmp_path in all steam_backlog_enforcer tests
- Add missing uninstall_other_games mock to 4 tests in
test_scanning.py (test_picks_shortest, test_skips_finished,
test_unknown_hours, test_picks_game_no_hours)
