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

package.json

@@ -57,9 +57,12 @@
     "@langchain/core": "^0.2.0",
     "@langchain/ollama": "^0.1.0",
     "@langchain/openai": "^0.1.0",
+    "@langfuse/otel": "^5.2.0",
+    "@langfuse/tracing": "^5.2.0",
     "@mcp-use/inspector": "^2.1.0",
     "@modelcontextprotocol/sdk": "^1.0.4",
     "@opentelemetry/api": "^1.9.1",
+    "@opentelemetry/sdk-node": "^0.215.0",
     "acorn": "^8.16.0",
     "cheerio": "^1.2.0",
     "commander": "^14.0.3",