Restore src.start_task entrypoint referenced by docs

[mkimhi]

Fixes #212.

docs/Entrance_en.md and the README instruct running python -m src.start_task -a, but src/start_task.py is missing on the current main branch.

This PR restores src.start_task (based on the earlier v0.2 implementation) so users can start task workers from configs/start_task.yaml again.

No benchmark logic is changed; this only restores the documented entrypoint and adds a brief clarification in the docs.

[Xiao9905]

@zhc7 please review

[paksas]

I tried applying the fix manually, and running the code yielded further errors:

/AgentBench/.venv/bin/python: No module named src.server.task_controller
AgentBench/.venv/bin/python: No module named src.server.task_worker
/AgentBench/.venv/bin/python: No module named src.server.task_worker

Would you mind sharing why this and the relevant files were deleted ? Was this a mistake, or perhaps a result of a merge going wrong ?