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_cli.py

@@ -113,6 +113,74 @@ def test_cost_estimate_cli(capsys):
     assert payload["cost_eur"] == 0.018
 
 
+def test_balance_cli_json(monkeypatch, capsys):
+    from llm_connect.balance import AccountBalance
+
+    def fake_get_balance(provider=None, *, registry=None, fx=None):
+        assert provider is None  # default path
+        return AccountBalance(
+            provider="openrouter",
+            currency="USD",
+            source="openrouter:/api/v1/credits+auth/key",
+            credits_remaining=12.5,
+            credits_remaining_eur=11.5,
+            limit_remaining=9.5,
+            limit_remaining_eur=8.74,
+            limit=10.0,
+            limit_reset="monthly",
+            fx_source="explicit",
+        )
+
+    monkeypatch.setattr("llm_connect.cli.get_account_balance", fake_get_balance)
+    assert main(["balance", "--json"]) == 0
+    payload = json.loads(capsys.readouterr().out)
+    assert payload["provider"] == "openrouter"
+    assert payload["credits_remaining"] == 12.5
+    assert payload["limit_remaining"] == 9.5
+
+
+def test_balance_cli_explicit_provider_one_shot(monkeypatch, capsys):
+    from llm_connect.balance import AccountBalance
+    from llm_connect.config import LLMConfig
+    from llm_connect.openrouter import _DEFAULT_MODEL
+
+    seen = {}
+
+    def fake_get_balance(provider=None, *, registry=None, fx=None):
+        seen["provider"] = provider
+        return AccountBalance(
+            provider="openrouter",
+            currency="USD",
+            source="test",
+            credits_remaining=1.0,
+        )
+
+    before_provider = LLMConfig().provider
+    before_model = _DEFAULT_MODEL
+    monkeypatch.setattr("llm_connect.cli.get_account_balance", fake_get_balance)
+    assert main(["balance", "--provider", "openrouter"]) == 0
+    assert seen["provider"] == "openrouter"
+    assert LLMConfig().provider == before_provider
+    assert _DEFAULT_MODEL == before_model
+    assert "account credits remaining" in capsys.readouterr().out
+
+
+def test_balance_cli_unsupported_provider(monkeypatch, capsys):
+    from llm_connect.exceptions import LLMBalanceUnsupportedError
+
+    def boom(provider=None, *, registry=None, fx=None):
+        raise LLMBalanceUnsupportedError(
+            "not supported",
+            provider=provider or "",
+            supported=["openrouter"],
+        )
+
+    monkeypatch.setattr("llm_connect.cli.get_account_balance", boom)
+    assert main(["balance", "--provider", "gemini"]) == 2
+    err = capsys.readouterr().err
+    assert "not supported" in err
+
+
 def test_spend_week_current_and_last(tmp_path, capsys, monkeypatch):
     ledger_path = tmp_path / "usage.jsonl"
     ledger = UsageLedger(ledger_path)