xrpc: add ListComments and UpdateComment methods

internal/atproto/xrpc/client.go

···
 	return nil
 }
 
-const collectionDocument = "com.diffdown.document"
-
 // GetDocument fetches a document by its rkey.
 func (c *Client) GetDocument(rkey string) (*model.Document, error) {
-	value, _, err := c.GetRecord(c.session.DID, collectionDocument, rkey)
+	value, _, err := c.GetRecord(c.session.DID, model.CollectionDocument, rkey)
 	if err != nil {
 		return nil, err
 	}
···
 
 // PutDocument creates or updates a document.
 func (c *Client) PutDocument(rkey string, doc *model.Document) (string, string, error) {
-	return c.PutRecord(collectionDocument, rkey, doc)
+	return c.PutRecord(model.CollectionDocument, rkey, doc)
+}
+
+// ListComments lists all comment records for a given document.
+func (c *Client) ListComments(did, docRKey string, limit int, cursor string) ([]Record, string, error) {
+	records, nextCursor, err := c.ListRecords(did, model.CollectionComment, limit, cursor)
+	if err != nil {
+		return nil, "", err
+	}
+	var filtered []Record
+	for _, r := range records {
+		var comment model.CommentRecord
+		if err := json.Unmarshal(r.Value, &comment); err != nil {
+			continue
+		}
+		if comment.DocRKey == docRKey {
+			filtered = append(filtered, r)
+		}
+	}
+	return filtered, nextCursor, nil
+}
+
+// UpdateComment updates an existing comment record (e.g., for resolution toggle).
+func (c *Client) UpdateComment(rkey string, record interface{}) (uri, cid string, err error) {
+	return c.PutRecord(model.CollectionComment, rkey, record)
 }