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-deactivator.php

@@ -0,0 +1,36 @@
+<?php
+/**
+ * Plugin Deactivation Handler
+ *
+ * Handles plugin deactivation logic.
+ *
+ * @package DDHH_Job_Manager
+ * @since 1.0.0
+ */
+
+// Exit if accessed directly.
+defined( 'ABSPATH' ) || exit;
+
+/**
+ * Class DDHH_JM_Deactivator
+ *
+ * Handles plugin deactivation.
+ */
+class DDHH_JM_Deactivator {
+
+	/**
+	 * Deactivate the plugin.
+	 *
+	 * Runs on plugin deactivation. Cleans up transients and flushes rewrite rules.
+	 * Does NOT delete user data as the user may reactivate the plugin.
+	 *
+	 * @since 1.0.0
+	 */
+	public static function deactivate() {
+		// Remove the rewrite flush transient if it exists.
+		delete_transient( 'ddhh_jm_flush_rewrite_rules' );
+
+		// Flush rewrite rules to clean up any custom rules added by the plugin.
+		flush_rewrite_rules();
+	}
+}