agent: Add setting to control edit card expanded state (#34040)

This PR adds the `expand_edit_card` setting, which controls whether edit
cards in the agent panel are expanded, thus showing or not the full diff
of a given file's AI-driven change. I personally prefer to have these
cards collapsed by default as I am mostly reviewing diffs using either
the review multibuffer or the diffs within the file's buffer itself.
Didn't want to change the default behavior as that was intentionally
chosen, so here we are! :)

Open to feedback about the setting name; I've iterated between a few
options and don't necessarily feel like the current one is the best.

Release Notes:

- agent: Added a setting to control whether edit cards are expanded in
the agent panel, thus showing or hiding the full diff of a file's
changes.

assets/settings/default.json

@@ -857,7 +857,11 @@
     // its response, or needs user input.
 
     // Default: false
-    "play_sound_when_agent_done": false
+    "play_sound_when_agent_done": false,
+    /// Whether to have edit cards in the agent panel expanded, showing a preview of the full diff.
+    ///
+    /// Default: true
+    "expand_edit_card": true
   },
   // The settings for slash commands.
   "slash_commands": {