refactor(sdk): move SDKCaptureHook to agent/hook.py

Colocate the capture hook with the rest of the hook infrastructure
instead of inlining it in the top-level facade module.

nanobot/agent/hook.py

@@ -102,3 +102,22 @@ class CompositeHook(AgentHook):
         for h in self._hooks:
             content = h.finalize_content(context, content)
         return content
+
+
+class SDKCaptureHook(AgentHook):
+    """Record tool names and the final message list for ``RunResult``.
+
+    The runner mutates ``context.messages`` in place across iterations, so the
+    snapshot is refreshed on every ``after_iteration`` call; the last call
+    reflects the end-of-turn state the SDK caller cares about.
+    """
+
+    def __init__(self) -> None:
+        super().__init__()
+        self.tools_used: list[str] = []
+        self.messages: list[dict[str, Any]] = []
+
+    async def after_iteration(self, context: AgentHookContext) -> None:
+        for call in context.tool_calls:
+            self.tools_used.append(call.name)
+        self.messages = list(context.messages)