feat(agent): read_window_below tool — which OS window is underneath the desktop app

Desktop-gated (desktop_ui toolset) metadata-only window awareness: the agent
can ask which application window sits directly behind the Hermes window
(app, title, bounds — never pixels). Rides the same blocking bridge as
read_terminal: the gateway emits window.read.request and the renderer
answers window.read.respond.

toolsets.py

@@ -273,6 +273,7 @@ TOOLSETS = {
         "tools": [
             "read_terminal", "close_terminal",
             "open_preview", "read_preview",
+            "read_window_below",
             "focus_pane", "react_to_message",
         ],
         "includes": []