Add test users to Playground demo blueprint

Creates Alice (author), Bob (author), and Carol (editor) so there
are users available to add as co-authors when testing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dd32

README.md

@@ -2,7 +2,7 @@
 
 A WordPress plugin that allows multiple authors to edit a single post via shared invite links. Compatible with WordPress collaborative editing.
 
-[Test in WordPress Playground](https://playground.wordpress.net/#eyJsYW5kaW5nUGFnZSI6Ii93cC1hZG1pbi9wb3N0LW5ldy5waHAiLCJsb2dpbiI6dHJ1ZSwic3RlcHMiOlt7InN0ZXAiOiJpbnN0YWxsUGx1Z2luIiwicGx1Z2luRGF0YSI6eyJyZXNvdXJjZSI6ImdpdDpkaXJlY3RvcnkiLCJ1cmwiOiJodHRwczovL2dpdGh1Yi5jb20vZGQzMi9tdWx0aS1hdXRob3ItcG9zdHMiLCJyZWYiOiJIRUFEIiwicmVmVHlwZSI6InJlZm5hbWUifX1dfQ==)
+[Test in WordPress Playground](https://playground.wordpress.net/#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)
 
 ## Features