Implement LLM-WP-0008: provider-scoped account balance CLI

Add pluggable balance registry, OpenRouter credits/key limit client,
and llm-connect balance with one-shot --provider that does not change
library defaults.

tests/test_package_exports.py

@@ -79,3 +79,20 @@ def test_wp_0007_spend_primitives_are_exported_from_package_root():
     for name in expected_names:
         assert hasattr(llm_connect, name)
         assert name in llm_connect.__all__
+
+
+def test_wp_0008_balance_primitives_are_exported_from_package_root():
+    expected_names = [
+        "AccountBalance",
+        "BalanceClientRegistry",
+        "OpenRouterBalanceClient",
+        "ProviderBalanceClient",
+        "format_balance_human",
+        "get_account_balance",
+        "resolve_balance_provider",
+        "LLMBalanceUnsupportedError",
+    ]
+
+    for name in expected_names:
+        assert hasattr(llm_connect, name)
+        assert name in llm_connect.__all__