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/exceptions.py

@@ -90,6 +90,30 @@ class LLMBudgetExceededError(LLMError):
         self.requested = requested
 
 
+class LLMBalanceUnsupportedError(LLMConfigurationError):
+    """Account balance is not supported for the selected provider.
+
+    Attributes:
+        provider: Provider id that was requested.
+        supported: Provider ids that currently implement balance.
+    """
+
+    def __init__(
+        self,
+        message: str,
+        provider: str = "",
+        supported: Optional[list[str]] = None,
+        cause: Optional[Exception] = None,
+        context: Optional[Dict[str, Any]] = None,
+    ):
+        supported_list = list(supported or [])
+        if context is None:
+            context = {"provider": provider, "supported": supported_list}
+        super().__init__(message, cause=cause, context=context)
+        self.provider = provider
+        self.supported = supported_list
+
+
 class LLMSubprocessError(LLMError):
     """Claude Code CLI subprocess failed.