Tracing: carry span into insight panic-isolation tasks; root the watcher boot probe (#143)

Found by the parity capture: both insight-generation handlers run the
generation inside a nested tokio::task::spawn (panic isolation) that was
not instrumented, so the background ai.insight.* trace still split off
from its 202 request. The watcher's boot-time availability probe ran
before the first tick span, leaving its count_for_library DB spans as
roots; it now opens `watcher.initial_probe`.

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

src/ai/handlers.rs

@@ -503,7 +503,8 @@ pub async fn generate_insight_handler(
 
             let path_for_task = path.clone();
             let generator_for_task = generator.clone();
-            let result = tokio::task::spawn(async move {
+            let result = tokio::task::spawn(
+                async move {
                     // Cross-model barrier: if a TTS synthesis holds the GPU, wait it
                     // out BEFORE the generation wall-clock starts. The per-request
                     // lease keeps reqwest budgets honest, but this job-level timeout
@@ -525,7 +526,10 @@ pub async fn generate_insight_handler(
                         ),
                     )
                     .await
-            })
+                }
+                // Inner panic-isolation task: carry the job span in too.
+                .instrument(tracing::Span::current()),
+            )
             .await;
 
             let mut dao = job_dao.lock().expect("Unable to lock InsightJobDao");
@@ -883,7 +887,8 @@ pub async fn generate_agentic_insight_handler(
 
             let path_for_task = path.clone();
             let generator_for_task = generator.clone();
-            let result = tokio::task::spawn(async move {
+            let result = tokio::task::spawn(
+                async move {
                     // Cross-model barrier — see generate_insight_handler: wait out any
                     // running TTS synthesis before the generation wall-clock starts.
                     drop(crate::ai::gpu::llm_lease().await);
@@ -908,7 +913,10 @@ pub async fn generate_agentic_insight_handler(
                         ),
                     )
                     .await
-            })
+                }
+                // Inner panic-isolation task: carry the job span in too.
+                .instrument(tracing::Span::current()),
+            )
             .await;
 
             let mut dao = job_dao.lock().expect("Unable to lock InsightJobDao");

src/watcher.rs

@@ -394,6 +394,7 @@ pub fn watch_files(
         // Disabled libraries skip the probe entirely — they should
         // never enter the health map (treated as out-of-scope).
         {
+            let _probe = tracing::info_span!("watcher.initial_probe").entered();
             let libs = libs_lock.read().unwrap_or_else(|e| e.into_inner());
             for lib in libs.iter() {
                 if !lib.enabled {