Expand AI tutorial with R script exercises

[JacobKhay]

Summary

• replace the unfinished Copilot placeholder with eight guided AI exercises using the terminal-based Codex assistant bundled in the current Codespace
• teach students to create and verify an R script, then run it with the Run Source button, Cmd/Ctrl + Shift + S, a hand-written source() call, and AI-driven Rscript
• demonstrate the current-session versus separate-process behavior and reinforce independent verification of AI edits and commands

Testing

• format_tutorial() idempotence check
• installed tutorial defaults validation and full tutorial render
• R CMD check --no-manual (Status: OK)