feat(langfuse): add opt-in Langfuse observability integration

Add Langfuse as an OTel trace transport for MCP request lifecycle tracking.

Changes:
- New src/lib/langfuse-transport.ts: initializes LangfuseSpanProcessor when
  ASK262_LANGFUSE_ENABLED=true with valid credentials
- src/lib/tracing.ts: add metadata to TraceContext, propagate it to child spans,
  use startActiveSpan for proper OTel context, add setupTracing() entry point,
  add getSessionMetadata() for version/transport metadata
- src/lib/logger.ts: include TraceContext.metadata in JSON log mixin
- src/mcp-server-{http,stdio}.ts: call setupTracing() at startup, pass
  version/transport metadata through withSpan/withSpanContext
- src/lib/fireworks-embeddings.ts: wrap API calls in withSpan() with gen_ai
  attributes for Langfuse generation/cost tracking
- .env.example: add LANGFUSE_* configuration variables
- AGENTS.md: document Langfuse integration
- package.json: add @langfuse/tracing, @langfuse/otel, @opentelemetry/sdk-node

src/lib/langfuse-transport.ts

@@ -0,0 +1,32 @@
+import { NodeTracerProvider } from "@opentelemetry/sdk-trace-node";
+import { LangfuseSpanProcessor } from "@langfuse/otel";
+
+let provider: NodeTracerProvider | null = null;
+
+/**
+ * Initialize Langfuse OTel span processor.
+ * Called once at server startup before any spans are created.
+ */
+export function initializeLangfuseTransport(): void {
+  if (process.env.ASK262_LANGFUSE_ENABLED !== "true") return;
+
+  const publicKey = process.env.LANGFUSE_PUBLIC_KEY;
+  const secretKey = process.env.LANGFUSE_SECRET_KEY;
+
+  if (!publicKey || !secretKey) {
+    console.error(
+      "[LANGFUSE] Missing LANGFUSE_PUBLIC_KEY or LANGFUSE_SECRET_KEY, skipping",
+    );
+    return;
+  }
+
+  provider = new NodeTracerProvider({
+    spanProcessors: [
+      new LangfuseSpanProcessor({
+        // Export all spans, not just LLM-relevant ones
+        shouldExportSpan: () => true,
+      }),
+    ],
+  });
+  provider.register();
+}