refactor and add list_snippets

lib/cache/snippets.ts

@@ -0,0 +1,7 @@
+import type { CodeSnippet } from "../types/index.js";
+
+/**
+ * In-memory cache to store snippets by ID for efficient retrieval
+ * Used by list_snippets and fetch_snippet_by_id commands
+ */
+export const snippetsCache: Record<string, CodeSnippet> = {};