doc update

GEMINI.md

@@ -240,6 +240,7 @@ For example, given a general `attachment` table containing a `kind` column (e.g.
   "type": "attachment",
   "properties": {
     "kind": {
+      "type": "string",
       "const": "cover"
     }
   }
@@ -252,10 +253,7 @@ A parent entity can then define a relationship using this constrained schema und
 "cover_attachment": {
   "properties": {
     "cover_attachment": {
-      "type": [
-        "cover.attachment",
-        "null"
-      ]
+      "type": "cover.attachment"
     }
   }
 }