Fix missing RowsAffected checks in cookie update methods

Address PR review comments: capture Exec result and check
RowsAffected() == 0 to return ErrResourceNotFound in
Cookie.Update, CookieCategory.Update, and CookieCategory.UpdateRank.
Also update coredata and relay contributor docs accordingly.

Signed-off-by: Émile Ré <emile@getprobo.com>

contrib/claude/coredata.md

@@ -129,7 +129,13 @@ assets, err := pgx.CollectRows(rows, pgx.RowToAddrOfStructByName[Asset])
 *a = assets
 
 // Update / Delete — no RETURNING
-_, err := conn.Exec(ctx, q, args)
+result, err := conn.Exec(ctx, q, args)
+if err != nil {
+    return err
+}
+if result.RowsAffected() == 0 {
+    return ErrResourceNotFound
+}
 ```
 
 ## Sentinel errors

contrib/claude/relay.md

@@ -341,7 +341,7 @@ Relay directives handle connection updates automatically — no manual store man
 
 #### Connection setup
 
-Any connection that a mutation will add to or remove from **must** have a `@connection` directive and expose `__id`:
+Any connection that a mutation will add to or remove from **must** have a `@connection` directive. If the mutation needs the connection ID in the same fragment, expose `__id`; otherwise derive it with `ConnectionHandler.getConnectionID`:
 
 ```tsx
 const fragment = graphql`