$ claude -p "Use the code-reviewer agent to review my changes"

**Review result: the diff is correct.** `shop/cart.py:24` now multiplies by `item.quantity`, which matches `tests/test_cart.py:9-13`. Nothing wrong in the change itself.

Pre-existing gaps it flagged (all out of scope for this fix):
- `shop/cart.py:11` — `quantity` unvalidated, so negative values silently reduce the total
- `shop/cart.py:20-21` — duplicate SKUs aren't merged (existing TODO)
- `shop/cart.py:29-31` — discount code is exact-match only (existing TODO)

Ship the diff. Fix the quantity validation when someone can actually submit a negative quantity — the rest are TODOs already tracked.
