cd15478-demos-and-exercises

Build a hands-on Python coding exercise for a Udacity Decision Science / Applied Statistics course module in the cd15478-demos-and-exercises repo. Use whenever the user wants to create an exercise from a module dictionary entry, build a starter + solution notebook pair for a specific learning objective, set up a new `module-name/exercise-name-starter/` folder, write `INSTRUCTIONS.md` for a coding exercise, or asks for a Jupyter exercise tied to a real-world dataset. Trigger eagerly — for example, when the user mentions "build an exercise", "create an exercise", "make an exercise for [module name]", references a "module dictionary" or "implementation module", points at a PDF of modules, or asks for help writing a coding exercise with starter and solution. Also trigger when the user is working inside or near the cd15478-demos-and-exercises repo and the task involves exercise authoring, even if they don't say the word "skill".