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/reels/script.rs

@@ -281,9 +281,8 @@ pub async fn generate_script(
     beats: &[PlannedBeat],
 ) -> Result<ReelScript> {
     let (system, user) = build_script_messages(meta, beats);
-    let cx = opentelemetry::Context::current();
     let raw = client
-        .generate(&cx, &user, Some(&system), None)
+        .generate(&user, Some(&system), None)
         .await
         .context("LLM script generation failed")?;
     Ok(parse_script_response(&raw, beats.len()))