feat: implement skeleton phase

src/beaver_gateway/frontends/base.py

@@ -0,0 +1,24 @@
+"""Frontend ABC.
+
+A frontend is anything that listens on a port and routes inbound traffic
+into the agent/MCP registries. ``configure`` is called once after the
+``Gateway`` is built; ``serve`` runs the listening loop.
+"""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from beaver_gateway.core.registry import Gateway
+
+
+class Frontend(ABC):
+    """Listens on a port, dispatches into the gateway."""
+
+    @abstractmethod
+    def configure(self, gateway: Gateway) -> None: ...
+
+    @abstractmethod
+    async def serve(self) -> None: ...