Add setting for enabling/disabling feedback (#30448)

This is useful for enterprises, especially in combination with #30444,
to ensure code never gets sent to Zed.

Release Notes:

- N/A

assets/settings/default.json

@@ -756,6 +756,8 @@
     "stream_edits": false,
     // When enabled, agent edits will be displayed in single-file editors for review
     "single_file_review": true,
+    // When enabled, show voting thumbs for feedback on agent edits.
+    "enable_feedback": true,
     "default_profile": "write",
     "profiles": {
       "write": {