[PB] Integrate the monitor into PBTask.grade

[thesofakillers]

Previously, users were expected to separately run the monitor posthoc, and adjusting the scores manually. This PR makes it so that the monitor is integrated into the grading process.

1. We assume the solver produces an agent.log at the root of the run_dir (this is currently not the case for aisi basic agent but we are working on a PR to fix this. DummySolver supports this.)
2. If the agent.log is available, we run the monitor on it before running reproduce.sh, if not we dont run the monitor. We mark grades with monitor_ran: bool to keep track of this
3. If the monitor picks up violations, we exit early and assign a grade of 0 for that run