feat(mcp) : add stdio auth, dashboard stats PoC tool, and helper trait

- McpStdioAuthSubscriber for console transport auth via env vars
- DashboardStatsTool as PoC (validates MCP protocol flow)
- McpToolHelper trait for shared pagination/error utilities
- Key learning: #[McpTool] must be on CLASS, not method for __invoke

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Mcp/Tool/DashboardStatsTool.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Mcp\Tool;
+
+use App\Repository\ComposantRepository;
+use App\Repository\MachineRepository;
+use App\Repository\PieceRepository;
+use App\Repository\ProductRepository;
+use App\Repository\SiteRepository;
+use Doctrine\ORM\EntityManagerInterface;
+use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Content\TextContent;
+
+#[McpTool(
+    name: 'get_dashboard_stats',
+    description: 'Get global inventory statistics: count of machines, pieces, composants, products, sites, and unresolved comments. Takes no parameters.',
+)]
+class DashboardStatsTool
+{
+    public function __construct(
+        private readonly MachineRepository $machines,
+        private readonly PieceRepository $pieces,
+        private readonly ComposantRepository $composants,
+        private readonly ProductRepository $products,
+        private readonly SiteRepository $sites,
+        private readonly EntityManagerInterface $em,
+    ) {}
+
+    public function __invoke(): array
+    {
+        $unresolvedComments = (int) $this->em->createQuery(
+            "SELECT COUNT(c.id) FROM App\\Entity\\Comment c WHERE c.status = 'open'"
+        )->getSingleScalarResult();
+
+        return [
+            new TextContent(
+                text: json_encode([
+                    'machines'           => $this->machines->count([]),
+                    'pieces'             => $this->pieces->count([]),
+                    'composants'         => $this->composants->count([]),
+                    'products'           => $this->products->count([]),
+                    'sites'              => $this->sites->count([]),
+                    'unresolvedComments' => $unresolvedComments,
+                ], JSON_THROW_ON_ERROR)
+            ),
+        ];
+    }
+}