feat(tools): tour — let the agent walk a user through the UI

One generic tool in the desktop_ui toolset: discover what is on screen,
highlight an element with narration, or hand the user a paged tour. No tour
content lives in the code — the agent authors each one live, which is what
makes 'how does this work?' answerable as a walkthrough instead of a wall
of text.

Rides the existing blocking-prompt bridge (tour.request/.respond) like
read_preview, so it works on every connection topology.

toolsets.py

@@ -279,7 +279,7 @@ TOOLSETS = {
             "open_preview", "read_preview",
             "read_window_below",
             "focus_pane", "react_to_message",
-            "setup_mcp",
+            "setup_mcp", "tour",
         ],
         "includes": []
     },