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

.env.example

@@ -62,3 +62,17 @@ ASK262_EMBEDDING_PROVIDER=ollama
 # Days to keep rotated logs before deletion (optional, defaults to 30)
 # Set to 0 to keep forever
 # ASK262_LOG_RETENTION_DAYS=30
+
+# =============================================================================
+# LANGFUSE OBSERVABILITY (opt-in)
+# =============================================================================
+
+# Enable Langfuse trace export (defaults to false)
+# ASK262_LANGFUSE_ENABLED=true
+
+# Langfuse credentials (required when enabled)
+# LANGFUSE_PUBLIC_KEY=pk-lf-...
+# LANGFUSE_SECRET_KEY=sk-lf-...
+
+# Langfuse host (optional, defaults to https://cloud.langfuse.com)
+# LANGFUSE_BASE_URL=https://cloud.langfuse.com