feat: implement G1 AI credits overage flow with billing telemetry

Adds end-to-end support for Google One AI credits in quota exhaustion flows:

- New billing module (packages/core/src/billing/) with credit balance
  checking, overage strategy management, and G1 URL construction
- OverageMenuDialog and EmptyWalletDialog UI components for quota
  exhaustion with credit purchase options
- Credits flow handler extracted to creditsFlowHandler.ts with overage
  menu, empty wallet, and auto-use-credits logic
- Server-side credit tracking: enabledCreditTypes on requests,
  consumed/remaining credits from streaming responses
- Billing telemetry events (overage menu shown, option selected, credits
  used, credit purchase click, API key updated)
- OpenTelemetry metrics for overage option and credit purchase counters
- Credit balance display in /stats command with refresh support
- Settings: general.overageStrategy (ask/always/never) for credit usage
- Error handling: INSUFFICIENT_G1_CREDITS_BALANCE as terminal error
  regardless of domain field presence
- Persistent info message after

packages/cli/src/config/settingsSchema.ts

@@ -828,6 +828,36 @@ const SETTINGS_SCHEMA = {
     ref: 'TelemetrySettings',
   },
 
+  billing: {
+    type: 'object',
+    label: 'Billing',
+    category: 'Advanced',
+    requiresRestart: false,
+    default: {},
+    description: 'Billing and AI credits settings.',
+    showInDialog: true,
+    properties: {
+      overageStrategy: {
+        type: 'enum',
+        label: 'Overage Strategy',
+        category: 'Advanced',
+        requiresRestart: false,
+        default: 'ask',
+        description: oneLine`
+          How to handle quota exhaustion when AI credits are available.
+          'ask' prompts each time, 'always' automatically uses credits,
+          'never' disables credit usage.
+        `,
+        showInDialog: true,
+        options: [
+          { value: 'ask', label: 'Ask each time' },
+          { value: 'always', label: 'Always use credits' },
+          { value: 'never', label: 'Never use credits' },
+        ],
+      },
+    },
+  },
+
   model: {
     type: 'object',
     label: 'Model',