Fix code quality issues from PR review

- Add matplotlib.use("Agg") backend configuration to prevent runtime errors in headless environments
- Add error handling guards for script results that might return None
- Fix AI targeting logic to exclude already-fired cells in super hunter and hunter modes
- Update CLAUDE.md with matplotlib best practices

research/activity24-math-plot.yaml

@@ -2,6 +2,8 @@ default_max_attempts_per_step: 3
 
 # Common processing script for all plotting steps
 common_processing_script: &plotting_script |
+  import matplotlib
+  matplotlib.use("Agg")
   import matplotlib.pyplot
   import numpy
   import io