feat: implement Phase 1 - Core Page Cache and Admin Settings UI

Implement the core page caching functionality and admin settings interface
for WP Recache plugin. This includes:

- Plugin bootstrap with activation/deactivation handlers
- Admin settings page with tabbed interface (Cache, Optimization, Advanced)
- Admin bar integration for quick cache purging
- Core page cache using output buffering
- Cache file management with mobile device separation
- Cache purge on post update/comment
- Default URL exclusions (cart, checkout, account)
- Cache statistics display
- PHPUnit test suite setup
- Docker-based development environment
- PHPStan configuration for static analysis

References:
- Closes #1 (Core Page Cache)
- Closes #8 (Admin Settings UI)

includes/Deactivator.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * Plugin deactivation handler.
+ *
+ * @package WP_Recache
+ */
+
+defined('ABSPATH') || exit;
+
+class WP_Recache_Deactivator {
+
+    /**
+     * Run deactivation routines.
+     */
+    public static function deactivate() {
+        self::remove_advanced_cache();
+        self::flush_rewrite_rules();
+    }
+
+    /**
+     * Remove advanced-cache.php drop-in.
+     */
+    private static function remove_advanced_cache() {
+        $advanced_cache = WP_CONTENT_DIR . '/advanced-cache.php';
+
+        if (file_exists($advanced_cache)) {
+            unlink($advanced_cache);
+        }
+    }
+
+    /**
+     * Flush rewrite rules.
+     */
+    private static function flush_rewrite_rules() {
+        flush_rewrite_rules();
+    }
+}