test(pricing): add vitest suite for pricing math and stacking invariants

- covers agent-request cost formula, weighted/value scores, budget filters,
  plan-model matching, apples-to-apples rows, stack candidate selection,
  Dave stacks and Mix & Match budget/distinctness properties
- run tests in the deploy workflow alongside lint and data validation

.github/workflows/deploy.yml

@@ -36,6 +36,9 @@ jobs:
       - name: Validate data invariants
         run: npm run validate-data
 
+      - name: Run unit tests
+        run: npm test
+
       - name: Build site
         run: npm run build