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.

llm_connect/cli.py

@@ -10,7 +10,12 @@ from datetime import datetime
 from pathlib import Path
 from typing import Any
 
+from llm_connect.balance import (
+    format_balance_human,
+    get_account_balance,
+)
 from llm_connect.costs import estimate_cost
+from llm_connect.exceptions import LLMBalanceUnsupportedError, LLMConfigurationError, LLMError
 from llm_connect.factory import create_adapter
 from llm_connect.fx import resolve_fx_rate
 from llm_connect.models import RunConfig
@@ -106,6 +111,30 @@ def _build_parser() -> argparse.ArgumentParser:
     spend_week.add_argument("--json", action="store_true", help="Emit JSON")
     spend_week.set_defaults(func=_spend_week)
 
+    balance = commands.add_parser(
+        "balance",
+        help=(
+            "Show prepaid/account remaining for a backend "
+            "(default: current default provider; --provider is one-shot only)"
+        ),
+    )
+    balance.add_argument(
+        "--provider",
+        default=None,
+        help=(
+            "Backend to query for this command only (does not change library defaults). "
+            "Omit to use the current default provider."
+        ),
+    )
+    balance.add_argument("--json", action="store_true", help="Emit JSON")
+    balance.add_argument(
+        "--eur-per-usd",
+        type=float,
+        default=None,
+        help="Override FX rate (euros per one USD)",
+    )
+    balance.set_defaults(func=_balance_show)
+
     return parser
 
 
@@ -340,6 +369,27 @@ def _fmt_local(dt: datetime) -> str:
     return dt.isoformat()
 
 
+def _balance_show(args: argparse.Namespace) -> int:
+    try:
+        balance = get_account_balance(args.provider, fx=args.eur_per_usd)
+    except LLMBalanceUnsupportedError as exc:
+        print(str(exc), file=sys.stderr)
+        return 2
+    except LLMConfigurationError as exc:
+        print(str(exc), file=sys.stderr)
+        return 2
+    except LLMError as exc:
+        print(str(exc), file=sys.stderr)
+        return 1
+
+    if args.json:
+        print(json.dumps(balance.to_dict(), indent=2, sort_keys=True))
+        return 0
+
+    print(format_balance_human(balance))
+    return 0
+
+
 def _classes_payload(classes: Iterable[ProblemClass]) -> dict[str, dict[str, Any]]:
     return {
         problem_class.name: {