Add pre-task baseline verifier check to warn on org contamination

[laurahanu]

Summary

Runs the SCUBA verifier against the starting org before each task begins and logs a warning if the baseline score is > 0. A clean org should score 0, so a non-zero baseline signals leftover artifacts from a previous run that could inflate the final score. This is a defensive layer to catch contamination before the agent starts.

• utils.py: shared run_baseline_check() helper; skips QA templates (they need an agent answer) and never raises so a check failure can't break the run
• args.py: --pretask_baseline_check flag (default on; --no- to disable)
• main_bu.py: check runs before agent.run()
• lib_eval_single_task.py: check runs in both CUA workers after env reset

Test plan

• Run a task on a deliberately dirtied org and confirm the warning fires
• Confirm QA-template tasks are skipped
• Confirm --no-pretask_baseline_check disables the check

[salesforce-cla]

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