agent: Add keybindings to open the panel's extra menu (#29663)

Maybe "extra" isn't the best word, but I'm referring to the ellipsis
menu on the far right of the panel that holds "extra" or "additional
options". There is a known issue with context menus, at least if
implemented this way, that makes hitting enter to select an option not
work. Will leave this fix to a later PR.

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -246,6 +246,7 @@
       "ctrl-alt-/": "assistant::ToggleModelSelector",
       "ctrl-shift-a": "agent::ToggleContextPicker",
       "ctrl-shift-o": "agent::ToggleNavigationMenu",
+      "ctrl-shift-i": "agent::ToggleOptionsMenu",
       "shift-escape": "agent::ExpandMessageEditor",
       "ctrl-e": "agent::ChatMode",
       "ctrl-alt-e": "agent::RemoveAllContext"