fix(billing): prefer Goose session cost over token estimate

Billing fell back to inflated token list-price estimates when Finish
frames carried inline accumulatedCost, causing ~70x overcharge vs DeepSeek.
Always fetch session accumulated_cost first; add Aug 4+ compensation script.

Co-authored-by: Cursor <cursoragent@cursor.com>

billing-token-state.test.mjs

@@ -23,7 +23,16 @@ test('estimateAccumulatedCostUsd uses DeepSeek-ish defaults', () => {
   assert.equal(estimate, 0.281);
 });
 
-test('enrichTokenStateForBilling prefers upstream cost over estimate', () => {
+test('enrichTokenStateForBilling prefers session cost over Finish inline cost', () => {
+  const enriched = enrichTokenStateForBilling(
+    { accumulatedInputTokens: 1000, accumulatedOutputTokens: 100, accumulatedCost: 0.82 },
+    { sessionCost: { accumulated_cost: 0.0067 } },
+    { H5_USE_BACKEND_COST: '1' },
+  );
+  assert.equal(enriched.accumulatedCost, 0.0067);
+});
+
+test('enrichTokenStateForBilling keeps Finish inline cost when session cost missing', () => {
   const enriched = enrichTokenStateForBilling(
     { accumulatedInputTokens: 1000, accumulatedOutputTokens: 100, accumulatedCost: 0.05 },
     {},
@@ -78,6 +87,22 @@ test('resolveBillingTokenState fetches session before estimating', async () => {
   assert.equal(resolved.accumulatedCost, 0.42);
 });
 
+test('resolveBillingTokenState replaces inflated Finish cost with session cost', async () => {
+  const resolved = await resolveBillingTokenState(
+    {
+      accumulatedInputTokens: 280030,
+      accumulatedOutputTokens: 5797,
+      accumulatedCost: 0.082,
+    },
+    {
+      sessionId: '20260804_19',
+      fetchSession: async () => ({ accumulated_cost: 0.006738208 }),
+    },
+    { H5_USE_BACKEND_COST: '1' },
+  );
+  assert.equal(resolved.accumulatedCost, 0.006738208);
+});
+
 test('enriched cost drives 1.2x billing instead of flat fallback', () => {
   const previous = { lastInputTokens: 224853, lastOutputTokens: 6685, lastAccumulatedCost: null };
   const tokenState = enrichTokenStateForBilling(