Add concrete Langfuse tracer and prompt gateway

src/core/langfuse/LangfusePromptGateway.ts

@@ -0,0 +1,25 @@
+import { Langfuse } from 'langfuse';
+import type { PromptFetcher, RawPrompt } from './PromptGateway';
+
+export class LangfusePromptGateway implements PromptFetcher {
+  private readonly client: Langfuse;
+
+  constructor(opts: { publicKey: string; secretKey: string; baseUrl: string }) {
+    this.client = new Langfuse({
+      publicKey: opts.publicKey,
+      secretKey: opts.secretKey,
+      baseUrl: opts.baseUrl,
+    });
+  }
+
+  async fetch(promptName: string): Promise<RawPrompt> {
+    const prompt = await this.client.getPrompt(promptName, undefined, { type: 'chat', cacheTtlSeconds: 0 });
+    return {
+      name: prompt.name,
+      version: prompt.version,
+      config: (prompt.config ?? {}) as Record<string, unknown>,
+      compile: (variables) =>
+        prompt.compile(variables) as { role: 'system' | 'user' | 'assistant'; content: string }[],
+    };
+  }
+}