feat(tools): close_preview so the agent can dismiss the pane it opened

open_preview and read_preview could drive the page, but nothing could close
the pane. Same desktop_ui / session-source gate as the rest of the GUI tools.

toolsets.py

@@ -264,7 +264,7 @@ TOOLSETS = {
     },
 
     # Affordances that only exist because a GUI renderer is on the other end of
-    # the connection: read/close the embedded terminal pane, open and read the
+    # the connection: read/close the embedded terminal pane, open/read/close the
     # in-app browser, focus a pane, tapback a message.
     #
     # Enabled by the GUI gateway for a session whose SOURCE is the desktop app
@@ -276,7 +276,7 @@ TOOLSETS = {
         "description": "Desktop GUI affordances — in-app terminal/browser panes, pane focus, reactions (GUI sessions only)",
         "tools": [
             "read_terminal", "close_terminal",
-            "open_preview", "read_preview",
+            "open_preview", "close_preview", "read_preview",
             "read_window_below",
             "focus_pane", "react_to_message",
             "setup_mcp", "tour",