refactor(agents,core,frontends): typed kinds, prompts per kind, frontend agents as parameters

src/beaver_gateway/frontends/base.py

@@ -19,6 +19,7 @@ if TYPE_CHECKING:
 
     from beaver_gateway.backends.base import Backend
     from beaver_gateway.core.auth import TokenStore
+    from beaver_gateway.core.kinds import Kind
     from beaver_gateway.core.registry import AgentRegistry, McpRegistry
     from beaver_gateway.core.turn_record import TurnRecord
     from beaver_gateway.storage import Database
@@ -101,7 +102,7 @@ class Frontend(ABC):
     """
 
     name: str = ""
-    kinds: tuple[str, ...] = ()
+    kinds: tuple[Kind, ...] = ()
 
     @abstractmethod
     def configure(self, runtime: GatewayRuntime) -> None: ...
@@ -109,7 +110,7 @@ class Frontend(ABC):
     @abstractmethod
     async def serve(self) -> None: ...
 
-    def agent_for(self, kind: str) -> str | None:  # noqa: ARG002
+    def agent_for(self, kind: Kind) -> str | None:  # noqa: ARG002
         return None
 
     async def materialize(self, conv: Conversation) -> ConversationBinding | None:  # noqa: ARG002