feat(01-01): create main plugin file with header

- Add WordPress plugin header with metadata
- Define plugin constants (VERSION, PLUGIN_DIR, PLUGIN_URL, PLUGIN_FILE)
- Add security check for direct access
- Include main class file
- Initialize plugin on plugins_loaded hook

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

ddhh-job-manager.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * Plugin Name: Digital Dabei Job Manager
+ * Plugin URI: https://www.hamburg.de/digital-dabei
+ * Description: Closed job board for provider self-registration and mentor applications
+ * Version: 1.0.0
+ * Author: digital dabei Hamburg
+ * Author URI: https://www.hamburg.de/digital-dabei
+ * Text Domain: ddhh-job-manager
+ * Domain Path: /languages
+ * Requires at least: 6.0
+ * Requires PHP: 7.4
+ * License: GPL v2 or later
+ * License URI: https://www.gnu.org/licenses/gpl-2.0.html
+ *
+ * @package DDHH_Job_Manager
+ */
+
+// Exit if accessed directly.
+defined( 'ABSPATH' ) || exit;
+
+// Define plugin constants.
+define( 'DDHH_JM_VERSION', '1.0.0' );
+define( 'DDHH_JM_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
+define( 'DDHH_JM_PLUGIN_URL', plugin_dir_url( __FILE__ ) );
+define( 'DDHH_JM_PLUGIN_FILE', __FILE__ );
+
+// Include the main plugin class.
+require_once DDHH_JM_PLUGIN_DIR . 'includes/class-ddhh-job-manager.php';
+
+/**
+ * Initialize the plugin.
+ */
+function ddhh_jm_init() {
+	DDHH_JM_Job_Manager::get_instance();
+}
+add_action( 'plugins_loaded', 'ddhh_jm_init', 10 );