Otel: llm.request client spans + traceparent on LLM calls (#130)

Add OpenTelemetry tracing to every LLM client call so an insight/chat
turn is a single trace from HTTP entry to each downstream inference.

- LlmClient trait: thread a &opentelemetry::Context through all 5
  methods (generate, chat_with_tools, chat_with_tools_stream,
  generate_embeddings, describe_image), matching the DAO convention of
  explicit context threading rather than ambient Context::current().
- otel::start_llm_request_span: single shared helper that every
  provider (openrouter / llamacpp / ollama) funnels through, so the
  span shape is identical: name llm.request, SpanKind::Client,
  llm.provider / llm.operation / llm.model.
- Per-method spans in all three clients with the spec's operation enum
  (chat | chat_stream | embeddings | describe_image | generate); stream
  spans are owned and moved into the async_stream generator so they end
  when the response drains, not when headers arrive.
- llm.tokens.input / llm.tokens.output recorded on non-stream and
  stream completion; llm.cost_usd from usage.cost (OpenRouter);
  http.status_code + error status on failure.
- W3C traceparent injected on outgoing LLM HTTP requests (llama-swap,
  Ollama, OpenRouter) gated on span_context.is_valid() so debug
  no-op-tracer builds don't send an all-zero parent.
- Turn-level spans (ai.chat.turn.execute, ai.persona_chat.turn.execute)
  now carry llm.backend + llm.model; with_entry streaming paths thread
  the turn context (turn_cx) to child describe_image / agentic-loop
  calls instead of the ambient context.

clippy clean; 690 lib + 775 integration + 4 doc tests green.

src/knowledge.rs

@@ -859,7 +859,8 @@ async fn synthesize_merge<D: KnowledgeDao + 'static>(
 
     let ollama = app_state.ollama.clone();
     let model_used = ollama.primary_model.clone();
-    let proposed = match ollama.generate(&prompt, Some(system)).await {
+    let cx = opentelemetry::Context::current();
+    let proposed = match ollama.generate(&cx, &prompt, Some(system)).await {
         Ok(out) => {
             // Strip the framing models reach for even with explicit
             // "no preamble" guidance: leading "Merged description:"