docs: keep the tasks-app scope note honest across the course

[recklessop]

The starter README says tasks-app is the running example for Module 1 and Module 2. In reality it's used from M1 through the capstone (agent edits it in M4, worktrees drive it in M7, the review lab pastes a patch onto it in M10, etc). Tighten the note so it reflects the actual scope.

Demo PR: this exists to illustrate a review comment on a deletion for a blog post about reviewing AI code. It's a real change and safe to merge if desired, but no rush.

[recklessop]

Nit — the delete captures the intent, but the new one-liner collapses M2's role into just "safety net." That works for someone reading top-down, but folks who land here from a search for "undo the AI" might miss that this is the repo M2 has them practice git restore on. Two options: (1) keep the delete and reword the new sentence to name the practice explicitly ("safety net you practice with git restore"), or (2) split the sentence and keep an explicit "Module 2" callout. I'd lean (1) — same tightness, doesn't lose the anchor.