refactor(agents-graph): drop redundant `agent_finish_called` set

``agent_finish`` was setting ``inner[\"agent_finish_called\"] = True``
at the top of its body, but ``StrixOrchestrationHooks.on_tool_end``
already does this for ``agent_finish`` and ``finish_scan`` after the
tool returns. Doing it twice was harmless but suggested the flag's
ownership was ambiguous; the hook is the single source of truth.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

strix/tools/agents_graph/tools.py

@@ -557,7 +557,8 @@ async def agent_finish(
             default=str,
         )
 
-    inner["agent_finish_called"] = True
+    # ``agent_finish_called`` is set by ``StrixOrchestrationHooks.on_tool_end``;
+    # no need to set it here.
 
     parent_notified = False
     if report_to_parent: