feat: add billing and usage domain cores

contracts/billing/core-v1.json

@@ -0,0 +1,15 @@
+{
+  "version": 1,
+  "currency": "CNY",
+  "moneyUnit": "fen",
+  "statuses": { "insufficientBalance": 402, "idempotencyConflict": 409 },
+  "idempotencyKeys": { "charge": "job-charge:{jobId}", "refund": "job-refund:{jobId}" },
+  "quote": {
+    "quantityRounding": "ceil",
+    "ruleOrder": ["reqKeySpecificity", "conditionSpecificity", "priority"],
+    "tiedWinner": "ambiguity_error",
+    "tierStandardFactors": "multiply",
+    "tierMarkupMultipliers": "maximum"
+  },
+  "exemptions": { "billingDisabled": true, "mockProvider": true, "platformSuperAdminQuota": true }
+}