tdd: add tautological-test anti-pattern

Tests whose assertion is recomputed the way the code computes it pass by
construction and give zero confidence. Add it as a peer of the existing
implementation-coupling anti-pattern: a Philosophy principle, a per-cycle
checklist gate, and a BAD/GOOD example pair in tests.md. Includes a patch
changeset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

skills/engineering/tdd/tests.md

@@ -59,3 +59,19 @@ test("createUser makes user retrievable", async () => {
   expect(retrieved.name).toBe("Alice");
 });
 ```
+
+**Tautological tests**: Expected value restates the implementation, so the test passes by construction.
+
+```typescript
+// BAD: Expected value is recomputed the way the code computes it
+test("calculateTotal sums line items", () => {
+  const items = [{ price: 10 }, { price: 5 }];
+  const expected = items.reduce((sum, i) => sum + i.price, 0);
+  expect(calculateTotal(items)).toBe(expected);
+});
+
+// GOOD: Expected value is an independent, known literal
+test("calculateTotal sums line items", () => {
+  expect(calculateTotal([{ price: 10 }, { price: 5 }])).toBe(15);
+});
+```