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/local_llm.rs

@@ -47,24 +47,12 @@ impl LocalLlm {
     /// pick query vs document — retrieval models treat the two sides
     /// differently and an unmarked embed invites prefix-mismatch bugs.
     pub async fn embed_query(&self, text: &str) -> Result<Vec<f32>> {
-        super::embed_query(
-            &opentelemetry::Context::new(),
-            &self.ollama,
-            self.llamacpp.as_deref(),
-            text,
-        )
-        .await
+        super::embed_query(&self.ollama, self.llamacpp.as_deref(), text).await
     }
 
     /// Embed corpus text (applies `EMBED_DOCUMENT_PREFIX`).
     pub async fn embed_document(&self, text: &str) -> Result<Vec<f32>> {
-        super::embed_document(
-            &opentelemetry::Context::new(),
-            &self.ollama,
-            self.llamacpp.as_deref(),
-            text,
-        )
-        .await
+        super::embed_document(&self.ollama, self.llamacpp.as_deref(), text).await
     }
 
     /// Single-shot local text generation via the `LLM_BACKEND`-selected
@@ -72,23 +60,14 @@ impl LocalLlm {
     pub async fn generate(&self, prompt: &str, system: Option<&str>) -> Result<String> {
         if super::local_backend_is_llamacpp() {
             if let Some(lc) = self.llamacpp.as_deref() {
-                return <LlamaCppClient as LlmClient>::generate(
-                    lc,
-                    &opentelemetry::Context::new(),
-                    prompt,
-                    system,
-                    None,
-                )
-                .await;
+                return <LlamaCppClient as LlmClient>::generate(lc, prompt, system, None).await;
             }
             anyhow::bail!(
                 "LLM_BACKEND=llamacpp but LlamaCppClient is unconfigured — \
                  set LLAMA_SWAP_URL or switch to LLM_BACKEND=ollama"
             );
         }
-        self.ollama
-            .generate(&opentelemetry::Context::new(), prompt, system)
-            .await
+        self.ollama.generate(prompt, system).await
     }
 
     /// Label identifying which backend + model produces embeddings right