fix: support terminal-bench original-tasks layout

[ningzimu]

Summary

• make terminal-rl/data_utils/download.py tolerant to upstream terminal-bench layout changes
• download tbench_core from original-tasks on current main, while keeping tasks as a fallback for older layouts
• refactor the sparse checkout setup into a shared helper to avoid duplicating git clone logic

Problem

download_tbench_core() currently assumes the upstream repo stores core tasks under tasks/ on main.

That path no longer exists on the current laude-institute/terminal-bench main branch, which now exposes the core dataset under original-tasks/. As a result, the script fails with a missing-path error when users try to download tbench_core.

Verification

• python3 -m py_compile terminal-rl/data_utils/download.py
• DATASET_DIR=/tmp/openclaw_tbench_download_test python3 ... mod.download_tbench_core()
• confirmed the download succeeds from original-tasks and yields 241 task directories