fix: correct case-sensitive require_once paths and add AGENTS.md (#14)

* fix: correct case-sensitive require_once paths and add AGENTS.md for AI guidance

- Fix includes/plugin.php -> includes/Plugin.php in main plugin file
- Fix includes/core.php -> includes/Core.php in includes/Plugin.php
  (Linux case-sensitive filesystems fail silently on macOS; runtime bug on production)
- Fix .ai-workflows/folder-structure.md: update class files to PascalCase,
  remove references to non-existent admin/ subdirectories (images/, partials/,
  settings/, tools/) that caused read:file_not_found errors in AI sessions
- Add AGENTS.md with authoritative file map to reduce AI agent errors:
  edit:not_read_first, bash:other, edit:edit_stale_read, read:file_not_found

Resolves #13

* fix: markdown formatting in AGENTS.md (Codacy MD022/MD032)

wp-fix-plugin-does-not-exist-notices.php

@@ -32,7 +32,7 @@ if (!defined('WPINC')) {
 }
 
 // Load the main plugin class
-require_once plugin_dir_path(__FILE__) . 'includes/plugin.php';
+require_once plugin_dir_path(__FILE__) . 'includes/Plugin.php';
 
 // Initialize the plugin
 // This is a test change for our new workflow