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/unified_search.rs

@@ -211,7 +211,7 @@ pub async fn unified_search<TagD: TagDao>(
     log::info!("unified_search: translating with model={}", backend.model());
 
     let today = chrono::Utc::now().date_naive();
-    let sq = match translate_nl_query(backend.chat(), &nl, &tag_vocab, today, &ctx).await {
+    let sq = match translate_nl_query(backend.chat(), &nl, &tag_vocab, today).await {
         Ok(sq) => sq,
         Err(e) => {
             log::warn!("unified_search: translate_nl_query failed: {e:?}");