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

tests/test_claude_sdk_backend.py

@@ -24,7 +24,7 @@ from claude_agent_sdk import (
 )
 
 from beaver_gateway.agents.base import ExposedMcp
-from beaver_gateway.agents.claude import ClaudeAgent, ClaudeOptions
+from beaver_gateway.agents.claude import ClaudeAgent, ClaudeOptions, Prompts
 from beaver_gateway.backends.claude_sdk import (
     ClaudeSdkBackend,
     RunnerConfig,
@@ -369,7 +369,7 @@ async def test_prompt_sources_are_assembled(cwd: Path) -> None:
     backend = _backend(
         cwd,
         InMemorySessionStore(),
-        prompt_sources=(("role", cwd / "a.md"), cwd / "b.md"),
+        prompts=Prompts(deep=(("role", cwd / "a.md"), cwd / "b.md")),
     )
     await _drain(
         backend.complete(