Tracing: LLM clients + streaming spans on tracing, drop cx threading (#140)

LlmClient methods no longer take an OpenTelemetry Context. llm.request is
now a tracing span (otel.kind=client, llm.provider/operation/model) that
parents on the caller's ambient span; traceparent injection reads the
span's OTel context via OpenTelemetrySpanExt.

The src/ai feature spans (handlers, insight generation, chat, persona
chat, daily summaries, TTS) move to #[instrument] / info_span!, and every
LLM-bearing tokio::spawn carries its parent span with .instrument(): the
streaming chat turn (chat_turn_stream_inner), async chat / persona turns,
background insight jobs and TTS jobs. Sub-spans in the agentic loops
(ai.chat.loop, ai.agentic.loop) instrument each LLM / tool await.

DAO calls keep their explicit Context (derived from the tracing span)
until the DAO migration (#141).

tests/otel_llm_spans.rs pins nesting + traceparent for the blocking and
streaming paths across a spawn on a multi-thread runtime.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ai/nl_query.rs

@@ -259,13 +259,10 @@ pub async fn translate_nl_query(
     nl: &str,
     tag_vocab: &[(i32, String)],
     today: chrono::NaiveDate,
-    cx: &opentelemetry::Context,
 ) -> Result<StructuredQuery> {
     let system = build_system_prompt(tag_vocab, today);
     let messages = vec![ChatMessage::system(system), ChatMessage::user(nl)];
-    let (msg, _, _) = client
-        .chat_with_tools(cx, messages, Vec::<Tool>::new())
-        .await?;
+    let (msg, _, _) = client.chat_with_tools(messages, Vec::<Tool>::new()).await?;
     parse_response(&msg.content, tag_vocab)
 }