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

src/beaver_gateway/frontends/markdown/frontend.py

@@ -76,6 +76,7 @@ from beaver_gateway.frontends.markdown.mirror import FRONTEND, ChatMirror
 if TYPE_CHECKING:
     from collections.abc import AsyncIterator, Callable
 
+    from beaver_gateway.core.kinds import Kind
     from beaver_gateway.frontends.base import GatewayRuntime
     from beaver_gateway.storage.models import Conversation, ConversationBinding
 
@@ -173,7 +174,7 @@ class MarkdownFrontend(Frontend):
             raise RuntimeError(msg)
         return self._mirror
 
-    def agent_for(self, kind: str) -> str | None:
+    def agent_for(self, kind: Kind) -> str | None:
         return self.default_agent if kind == "deep" else None
 
     async def materialize(self, conv: Conversation) -> ConversationBinding | None: