feat(01-01): create main plugin class with activation/deactivation

- Add main singleton class DDHH_JM_Job_Manager
- Implement activation handler with WP/PHP version checks
- Implement deactivation handler with rewrite flush
- Use transient-based rewrite flush to avoid multiple flushes
- Add comprehensive security checks and documentation

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

includes/class-ddhh-job-manager.php

@@ -0,0 +1,103 @@
+<?php
+/**
+ * Main Plugin Class
+ *
+ * The core plugin class that orchestrates all functionality.
+ *
+ * @package DDHH_Job_Manager
+ * @since 1.0.0
+ */
+
+// Exit if accessed directly.
+defined( 'ABSPATH' ) || exit;
+
+/**
+ * Class DDHH_JM_Job_Manager
+ *
+ * Main plugin class using singleton pattern.
+ */
+class DDHH_JM_Job_Manager {
+
+	/**
+	 * The single instance of the class.
+	 *
+	 * @var DDHH_JM_Job_Manager|null
+	 */
+	private static $instance = null;
+
+	/**
+	 * Get the singleton instance.
+	 *
+	 * @since 1.0.0
+	 * @return DDHH_JM_Job_Manager
+	 */
+	public static function get_instance() {
+		if ( null === self::$instance ) {
+			self::$instance = new self();
+		}
+		return self::$instance;
+	}
+
+	/**
+	 * Private constructor to prevent direct instantiation.
+	 *
+	 * @since 1.0.0
+	 */
+	private function __construct() {
+		$this->register_hooks();
+		$this->check_rewrite_flush();
+	}
+
+	/**
+	 * Prevent cloning of the instance.
+	 *
+	 * @since 1.0.0
+	 */
+	private function __clone() {}
+
+	/**
+	 * Prevent unserializing of the instance.
+	 *
+	 * @since 1.0.0
+	 */
+	public function __wakeup() {
+		throw new Exception( 'Cannot unserialize singleton' );
+	}
+
+	/**
+	 * Register activation and deactivation hooks.
+	 *
+	 * @since 1.0.0
+	 */
+	private function register_hooks() {
+		// Load activator and deactivator classes.
+		require_once DDHH_JM_PLUGIN_DIR . 'includes/class-activator.php';
+		require_once DDHH_JM_PLUGIN_DIR . 'includes/class-deactivator.php';
+
+		// Register activation hook.
+		register_activation_hook(
+			DDHH_JM_PLUGIN_FILE,
+			array( 'DDHH_JM_Activator', 'activate' )
+		);
+
+		// Register deactivation hook.
+		register_deactivation_hook(
+			DDHH_JM_PLUGIN_FILE,
+			array( 'DDHH_JM_Deactivator', 'deactivate' )
+		);
+	}
+
+	/**
+	 * Check if rewrite rules need to be flushed.
+	 *
+	 * This is triggered by a transient set during activation.
+	 *
+	 * @since 1.0.0
+	 */
+	private function check_rewrite_flush() {
+		if ( get_transient( 'ddhh_jm_flush_rewrite_rules' ) ) {
+			flush_rewrite_rules();
+			delete_transient( 'ddhh_jm_flush_rewrite_rules' );
+		}
+	}
+}