agent: Improve consecutive tool call UX and rebrand Max Mode (#31470)

This PR improves the consecutive tool call UX by allowing users to
quickly continue an interrupted with one-click. What we do here is
insert a hidden "Continue" message that will just nudge the LLM to keep
going. We're also using the opportunity to upsell the previously called
"Max Mode", now rebranded as "Burn Mode", which allows users to don't be
interrupted anymore if they ever have 25 consecutive tool calls again.

Release Notes:

- agent: Improve consecutive tool call UX by allowing users to quickly
continue an interrupted thread with one click.

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>
Co-authored-by: Agus Zubiaga <hi@aguz.me>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

assets/keymaps/default-linux.json

@@ -248,7 +248,9 @@
       "ctrl-shift-i": "agent::ToggleOptionsMenu",
       "shift-alt-escape": "agent::ExpandMessageEditor",
       "ctrl-alt-e": "agent::RemoveAllContext",
-      "ctrl-shift-e": "project_panel::ToggleFocus"
+      "ctrl-shift-e": "project_panel::ToggleFocus",
+      "ctrl-shift-enter": "agent::ContinueThread",
+      "alt-enter": "agent::ContinueWithBurnMode"
     }
   },
   {