agent: Add keybinding to toggle Burn Mode (#31630)

One caveat with this PR is that the keybinding still doesn't work for text threads. Will do that in a follow-up.

Release Notes:

- agent: Added a keybinding to toggle Burn Mode on and off.

assets/keymaps/default-linux.json

@@ -250,7 +250,8 @@
       "ctrl-alt-e": "agent::RemoveAllContext",
       "ctrl-shift-e": "project_panel::ToggleFocus",
       "ctrl-shift-enter": "agent::ContinueThread",
-      "alt-enter": "agent::ContinueWithBurnMode"
+      "alt-enter": "agent::ContinueWithBurnMode",
+      "ctrl-alt-b": "agent::ToggleBurnMode"
     }
   },
   {

assets/keymaps/default-macos.json

@@ -285,7 +285,8 @@
       "cmd-alt-e": "agent::RemoveAllContext",
       "cmd-shift-e": "project_panel::ToggleFocus",
       "cmd-shift-enter": "agent::ContinueThread",
-      "alt-enter": "agent::ContinueWithBurnMode"
+      "alt-enter": "agent::ContinueWithBurnMode",
+      "cmd-alt-b": "agent::ToggleBurnMode"
     }
   },
   {