Document closed and column fields in card API response

Add missing fields to the GET card endpoint documentation:
- closed: indicates whether the card is in "Done" state
- column: present only when card is triaged into a column

docs/API.md

@@ -574,6 +574,7 @@ __Response:__
   "description_html": "<div class=\"action-text-content\"><p>Hello, World!</p></div>",
   "image_url": null,
   "tags": ["programming"],
+  "closed": false,
   "golden": false,
   "last_active_at": "2025-12-05T19:38:48.553Z",
   "created_at": "2025-12-05T19:38:48.540Z",
@@ -594,6 +595,15 @@ __Response:__
       "url": "http://fizzy.localhost:3006/897362094/users/03f5v9zjw7pz8717a4no1h8a7"
     }
   },
+  "column": {
+    "id": "03f5v9zkft4hj9qq0lsn9ohcn",
+    "name": "In Progress",
+    "color": {
+      "name": "Lime",
+      "value": "var(--color-card-4)"
+    },
+    "created_at": "2025-12-05T19:36:35.534Z"
+  },
   "creator": {
     "id": "03f5v9zjw7pz8717a4no1h8a7",
     "name": "David Heinemeier Hansson",
@@ -619,6 +629,8 @@ __Response:__
 }
 ```
 
+> **Note:** The `closed` field indicates whether the card is in the "Done" state. The `column` field is only present when the card has been triaged into a column; cards in "Maybe?", "Not Now" or "Done" will not have this field.
+
 ### `POST /:account_slug/boards/:board_id/cards`
 
 Creates a new card in a board.