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/bin/test_daily_summary.rs

@@ -220,7 +220,9 @@ async fn main() -> Result<()> {
 
         println!("Generating summary...");
 
-        let summary = ollama.generate(&prompt, Some(system_prompt)).await?;
+        let summary = ollama
+            .generate(&opentelemetry::Context::new(), &prompt, Some(system_prompt))
+            .await?;
 
         println!("\n📝 GENERATED SUMMARY:");
         println!("─────────────────────────────────────────");
@@ -236,7 +238,9 @@ async fn main() -> Result<()> {
             );
 
             println!("\nGenerating embedding...");
-            let embedding = ollama.generate_embedding(&stripped).await?;
+            let embedding = ollama
+                .generate_embedding(&opentelemetry::Context::new(), &stripped)
+                .await?;
             println!("✓ Embedding generated ({} dimensions)", embedding.len());
 
             println!("Saving to database...");