refactor: Cleanup git state - commit all staged changes

Major refactoring cleanup:
- Add new controller architecture (class-controller-*.php)
- Add new settings-v2 UI (views/settings-v2/)
- Add new CSS architecture (agentic-sidebar.css, tokens)
- Add esbuild build pipeline (scripts/build.js, package.json)
- Add composer dependencies (vendor/)
- Add frontend src directory (assets/js/src/index.jsx)
- Add documentation files
- Remove old/obsolete files (class-settings.php, old CSS)

This commits all pending changes from previous refactoring efforts.

includes/class-controller-cost.php

@@ -0,0 +1,63 @@
+<?php
+/**
+ * Cost REST Controller
+ *
+ * Handles cost tracking operations.
+ *
+ * @package WP_Agentic_Writer
+ */
+
+/**
+ * Class WP_Agentic_Writer_Controller_Cost
+ *
+ * REST controller for cost tracking operations.
+ *
+ * @since 0.3.0
+ */
+class WP_Agentic_Writer_Controller_Cost {
+
+    /**
+     * Sidebar instance for dependency access.
+     *
+     * @var WP_Agentic_Writer_Gutenberg_Sidebar
+     */
+    private $sidebar;
+
+    /**
+     * Constructor.
+     *
+     * @since 0.3.0
+     * @param WP_Agentic_Writer_Gutenberg_Sidebar $sidebar Sidebar instance.
+     */
+    public function __construct( $sidebar ) {
+        $this->sidebar = $sidebar;
+    }
+
+    /**
+     * Handle get cost tracking request.
+     *
+     * @since 0.1.0
+     * @param WP_REST_Request $request REST request.
+     * @return WP_REST_Response|WP_Error Response.
+     */
+    public function handle_get_cost_tracking( $request ) {
+        $post_id = $request->get_param( 'post_id' );
+
+        // Check post-specific permission if post_id is provided.
+        if ( $post_id > 0 && ! $this->sidebar->check_post_permission( $post_id ) ) {
+            return new WP_Error(
+                'forbidden',
+                __(
+                    'You do not have permission to access this post.',
+                    'wp-agentic-writer',
+                ),
+                [ 'status' => 403 ],
+            );
+        }
+
+        $cost_tracker = WP_Agentic_Writer_Cost_Tracker::get_instance();
+        $data          = $cost_tracker->get_frontend_data( $post_id );
+
+        return new WP_REST_Response( $data, 200 );
+    }
+}