feat(core/agent): 添加 ToolDispatcher Protocol 和 AgentLoopSpec hookspec

ToolDispatcher async + context 参数。AgentLoopSpec 四个 async 生命周期 hook。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/agent/protocols.py

@@ -0,0 +1,45 @@
+"""Agent 专属 Protocol 端口。"""
+from __future__ import annotations
+
+from typing import Any, Protocol, runtime_checkable
+
+import pluggy
+
+from core.agent.types import LoopResult, Step
+
+hookspec = pluggy.HookspecMarker("agent_loop")
+hookimpl = pluggy.HookimplMarker("agent_loop")
+
+
+@runtime_checkable
+class ToolDispatcher(Protocol):
+    """Agent 工具调度端口。无效工具名抛 ValueError。"""
+
+    async def dispatch(
+        self, tool_name: str, args: dict[str, Any], *, context: dict[str, Any]
+    ) -> str: ...
+
+
+class AgentLoopSpec:
+    """AgentLoop 生命周期扩展点。
+
+    每个 hookimpl 可选择观察（返回 None）或变换（返回值）。
+    """
+
+    @hookspec
+    async def before_step(
+        self, iteration: int, messages: list[dict[str, Any]]
+    ) -> None: ...
+
+    @hookspec
+    async def after_tool(
+        self, iteration: int, step: Step
+    ) -> str | None: ...
+
+    @hookspec
+    async def after_step(
+        self, iteration: int, messages: list[dict[str, Any]]
+    ) -> None: ...
+
+    @hookspec
+    async def on_finish(self, result: LoopResult) -> None: ...