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

src/beaver_gateway/backends/claude_sdk.py

@@ -77,6 +77,7 @@ from beaver_gateway.core.events import (
     build_thinking_delta,
     build_tool_use_block_start,
 )
+from beaver_gateway.core.kinds import as_kind
 from beaver_gateway.core.sessions import Session, SessionClient, SessionPool
 from beaver_gateway.core.transcript import (
     build_entries,
@@ -511,7 +512,7 @@ class ClaudeSdkBackend:
             env["HOME"] = str(self._runner.home)
             env.setdefault("CLAUDE_CONFIG_DIR", str(self._runner.home / ".claude"))
         plugins = self._plugins()
-        sources = agent.prompt_for(spec.kind)
+        sources = agent.prompt_for(as_kind(spec.kind))
         system_prompt = (
             prompt_assembly.assemble(sources) if sources else agent.system_prompt
         )