chore(06-02): download and include Action Scheduler library

- Downloaded Action Scheduler 3.9.3 from GitHub
- Placed in vendor/action-scheduler/ directory
- Included in main plugin file before other code for proper initialization
- Library will auto-initialize itself when required

ddhh-job-manager.php

@@ -19,6 +19,10 @@
 // Exit if accessed directly.
 defined( 'ABSPATH' ) || exit;
 
+// Load Action Scheduler before other code for proper initialization.
+// Action Scheduler must be included early so it can register its hooks and tables.
+require_once plugin_dir_path( __FILE__ ) . 'vendor/action-scheduler/action-scheduler.php';
+
 // Define plugin constants.
 define( 'DDHH_JM_VERSION', '1.0.0' );
 define( 'DDHH_JM_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );