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

docs/reference/configuration.md

@@ -357,6 +357,15 @@ their corresponding top-level category object in your `settings.json` file.
   - **Default:** `true`
   - **Requires restart:** Yes
 
+#### `billing`
+
+- **`billing.overageStrategy`** (enum):
+  - **Description:** How to handle quota exhaustion when AI credits are
+    available. 'ask' prompts each time, 'always' automatically uses credits,
+    'never' disables credit usage.
+  - **Default:** `"ask"`
+  - **Values:** `"ask"`, `"always"`, `"never"`
+
 #### `model`
 
 - **`model.name`** (string):