linux: Add agent::ToggleBurnMode shortcut (super-ctrl-b) (#33458)

Follow-up to: 
- https://github.com/zed-industries/zed/pull/33452
- https://github.com/zed-industries/zed/pull/33190
- https://github.com/zed-industries/zed/pull/31630

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -244,6 +244,7 @@
       "ctrl-alt-e": "agent::RemoveAllContext",
       "ctrl-shift-e": "project_panel::ToggleFocus",
       "ctrl-shift-enter": "agent::ContinueThread",
+      "super-ctrl-b": "agent::ToggleBurnMode",
       "alt-enter": "agent::ContinueWithBurnMode"
     }
   },