agent: Add use_modifier_to_send setting (#34709)

When `use_modifier_to_send` is turned on, holding `cmd`/`ctrl` is
necessary to send a message in the agent panel. Text threads already use
`cmd-enter` by default to submit a message, and it was done this way to
have the usual text editing bindings not taken over when writing a
prompt, sort of stimulating more thoughtful writing. While `enter` to
send is still somewhat a huge pattern in chat-like LLM UIs, it still
makes sense to allow this for the new agent panel... hence the existence
of this setting now!

Release Notes:

- agent: Added the `use_modifier_to_send` setting, which makes holding a
modifier (`cmd`/`ctrl`), together with `enter`, required to send a new
message.

assets/keymaps/default-linux.json

@@ -277,7 +277,7 @@
     }
   },
   {
-    "context": "MessageEditor > Editor",
+    "context": "MessageEditor > Editor && !use_modifier_to_send",
     "bindings": {
       "enter": "agent::Chat",
       "ctrl-enter": "agent::ChatWithFollow",
@@ -287,6 +287,17 @@
       "ctrl-shift-n": "agent::RejectAll"
     }
   },
+  {
+    "context": "MessageEditor > Editor && use_modifier_to_send",
+    "bindings": {
+      "ctrl-enter": "agent::Chat",
+      "enter": "editor::Newline",
+      "ctrl-i": "agent::ToggleProfileSelector",
+      "shift-ctrl-r": "agent::OpenAgentDiff",
+      "ctrl-shift-y": "agent::KeepAll",
+      "ctrl-shift-n": "agent::RejectAll"
+    }
+  },
   {
     "context": "EditMessageEditor > Editor",
     "bindings": {