feat: guide tool and server instructions

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Guides/GuideRepository.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Guides;
+
+/**
+ * Reads guide markdown files from one directory. Topic names are restricted to
+ * lowercase slugs, so no path outside the directory can ever be read.
+ */
+final class GuideRepository
+{
+    private const TOPIC_PATTERN = '/^[a-z0-9]+(?:-[a-z0-9]+)*$/';
+
+    public function __construct(private string $directory)
+    {
+    }
+
+    /** @return list<string> */
+    public function topics(): array
+    {
+        $files = glob($this->directory . '/*.md');
+        if ($files === false) {
+            return [];
+        }
+
+        $topics = [];
+        foreach ($files as $file) {
+            $topic = basename($file, '.md');
+            if (preg_match(self::TOPIC_PATTERN, $topic) === 1) {
+                $topics[] = $topic;
+            }
+        }
+        sort($topics);
+
+        return $topics;
+    }
+
+    public function get(string $topic): ?string
+    {
+        if (preg_match(self::TOPIC_PATTERN, $topic) !== 1 || ! in_array($topic, $this->topics(), true)) {
+            return null;
+        }
+
+        $path = $this->directory . '/' . $topic . '.md';
+        // phpcs:ignore WordPress.WP.AlternativeFunctions.file_get_contents_file_get_contents
+        $content = file_get_contents($path);
+
+        return $content === false ? null : $content;
+    }
+}