agent: Change the reject changes keybinding (#28381)

This PR makes the reject keybinding, in the Review Changes mutlbuffer,
`cmd-n`.

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -150,7 +150,7 @@
     "context": "AgentDiff",
     "bindings": {
       "ctrl-y": "agent::Keep",
-      "ctrl-k ctrl-r": "agent::Reject"
+      "ctrl-n": "agent::Reject"
     }
   },
   {

assets/keymaps/default-macos.json

@@ -242,7 +242,7 @@
     "use_key_equivalents": true,
     "bindings": {
       "cmd-y": "agent::Keep",
-      "cmd-alt-z": "agent::Reject"
+      "cmd-n": "agent::Reject"
     }
   },
   {