assistant: Rename action from New Context to New Chat (#25455)

If you looked that up via the Command Palette, we were showing an
outdated action name ("new context") which causes confusion given the
panel says "New Chat".

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -184,9 +184,9 @@
       "ctrl-alt-/": "assistant::ToggleModelSelector",
       "ctrl-k h": "assistant::DeployHistory",
       "ctrl-k l": "assistant::DeployPromptLibrary",
-      "new": "assistant::NewContext",
-      "ctrl-t": "assistant::NewContext",
-      "ctrl-n": "assistant::NewContext"
+      "new": "assistant::NewChat",
+      "ctrl-t": "assistant::NewChat",
+      "ctrl-n": "assistant::NewChat"
     }
   },
   {