feat: skip Stripe billing in evals by setting CI=true

Instead of adding a new SKIP_BILLING env var, evals now set CI=true at the
top of their entry points (main.ts, main-nightly.ts, main-single-eval.ts,
main-hard-tasks.ts). This ensures billing is always skipped when running
evals, without requiring users to remember to set an env var.

The shouldAttemptStripeMetering() function already checks CI=true, so this
approach is simpler and follows the existing pattern.

Author: brandonkachen

evals/buffbench/main-hard-tasks.ts

@@ -1,3 +1,6 @@
+// Set CI=true to skip Stripe billing during evals
+process.env.CI = 'true'
+
 import fs from 'fs'
 import path from 'path'
 

evals/buffbench/main-nightly.ts

@@ -1,3 +1,6 @@
+// Set CI=true to skip Stripe billing during evals
+process.env.CI = 'true'
+
 import path from 'path'
 
 import { sendBasicEmail } from '@codebuff/internal/loops'

evals/buffbench/main-single-eval.ts

@@ -1,3 +1,6 @@
+// Set CI=true to skip Stripe billing during evals
+process.env.CI = 'true'
+
 import path from 'path'
 
 import { runBuffBench } from './run-buffbench'

evals/buffbench/main.ts

@@ -1,3 +1,6 @@
+// Set CI=true to skip Stripe billing during evals
+process.env.CI = 'true'
+
 import path from 'path'
 
 import { runBuffBench } from './run-buffbench'

packages/billing/src/stripe-metering.ts

@@ -10,7 +10,8 @@ const STRIPE_METER_EVENT_NAME = 'credits'
 const STRIPE_METER_REQUEST_TIMEOUT_MS = 10_000
 
 function shouldAttemptStripeMetering(): boolean {
-  // Avoid sending Stripe metering events in CI and when Stripe isn't configured.
+  // Avoid sending Stripe metering events in CI or when Stripe isn't configured.
+  // Evals set CI=true to skip billing.
   if (process.env.CI === 'true' || process.env.CI === '1') return false
   return Boolean(process.env.STRIPE_SECRET_KEY)
 }