feat: implement claude code backend

src/beaver_gateway/frontends/base.py

@@ -11,10 +11,12 @@ and hands it to each frontend's ``configure``.
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
+    from collections.abc import Mapping
+
     from beaver_gateway.backends.base import Backend
     from beaver_gateway.core.auth import TokenStore
     from beaver_gateway.core.registry import AgentRegistry, McpRegistry
@@ -28,12 +30,18 @@ class GatewayRuntime:
     instance can serve many ``RaycastAgent`` instances, but the lookup
     site (an inbound request with ``model=<agent.name>``) already has
     the name in hand, so the indirection lives one step earlier.
+
+    ``mcp_internal_urls`` is filled in Phase 2.1: one loopback URL per
+    declared ``McpServer`` so ``ClaudeCodeBackendAdapter`` (Phase 2.2)
+    can pass them to ``BackendOptions.mcp_servers`` without re-running
+    discovery.
     """
 
     agents: AgentRegistry
     mcps: McpRegistry
     backends: dict[str, Backend]
     token_store: TokenStore
+    mcp_internal_urls: Mapping[str, str] = field(default_factory=dict)
 
 
 class Frontend(ABC):